Business Combinations - Catalyst Merger (Details) (USD $)
 0 Months Ended 12 Months Ended
Jul. 02, 2012
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Jul. 02, 2012
Merger Agreement [Abstract]          
Amortization of intangible assets   $ 213,220,000 $ 203,192,000 $ 130,116,000  
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]          
Goodwill   4,724,639,000 4,720,275,000 4,478,038,000  
Common Shares          
Merger Agreement [Abstract]          
Public offering, shares of common stock       11,960,000  
Catalyst          
Merger Agreement [Abstract]          
Goodwill expected to be tax deductible 525,000,000       525,000,000
Amortization of intangible assets   154,700,000 166,100,000    
Fair value of intangible assets 1,200,000,000        
Customer relationships, weighted average useful life   9 years      
Business Combination, Consideration Transferred [Abstract]          
Fair value of common shares issued 3,238,141,000        
Fair value of warrants and stock options issued 19,824,000        
Total purchase price 4,673,241,000        
Separate Tranactions and Preexisting Relationships [Abstract]          
Transaction and integration expense       27,200,000  
Business Combination, Transaction Expense       22,800,000  
Acquisition costs included in selling, general and administrative expenses       17,000,000  
Contract settlements and termination charges recorded       3,500,000  
Accounts receivable with acquired business       4,100,000  
Catalyst | Common Shares          
Merger Agreement [Abstract]          
Stock conversion, share received per share converted 1.3212       1.3212
Stock conversion, cash received per share converted, prior to stock split 0.6606       0.6606
Stock conversion, cash received per share converted $ 28.00       $ 28.00
Public offering, shares of common stock 66,800,000        
Warrants, Issued 500,000        
Business Combination, Consideration Transferred [Abstract]          
Cash paid to shareholders 1,415,276,000        
Catalyst | Initial Amounts Recognized at Acquisition Date          
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]          
Cash and cash equivalents 93,775,000       93,775,000
Other current assets 695,888,000       695,888,000
Total current assets 789,663,000       789,663,000
Goodwill 4,010,235,000       4,010,235,000
Customer relationships intangible 1,184,800,000       1,184,800,000
Other long-term assets 87,174,000       87,174,000
Total assets acquired 6,071,872,000       6,071,872,000
Accounts payable 338,819,000       338,819,000
Pharmacy benefit management rebates payable 176,202,000       176,202,000
Accrued expenses and other current liabilities 187,851,000       187,851,000
Long-term debt 311,994,000       311,994,000
Other long-term liabilities 385,375,000       385,375,000
Total liabilities assumed 1,400,241,000       1,400,241,000
Non-controlling interest (1,610,000)       (1,610,000)
Net assets acquired 4,673,241,000       4,673,241,000
Catalyst | Measurement Period Adjustments          
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]          
Cash and cash equivalents     0 (315,000)  
Other current assets     2,411,000 5,202,000  
Total current assets     2,411,000 4,887,000  
Goodwill     16,141,000 8,492,000  
Customer relationships intangible     0 0  
Other long-term assets     8,000 1,547,000  
Total assets acquired     18,560,000 14,926,000  
Accounts payable     5,000 0  
Pharmacy benefit management rebates payable     (1,522,000) 2,935,000  
Accrued expenses and other current liabilities     5,473,000 1,348,000  
Long-term debt     0 0  
Other long-term liabilities     14,604,000 10,643,000  
Total liabilities assumed     18,560,000 14,926,000  
Non-controlling interest     0 0  
Net assets acquired     0 0  
Catalyst | Final Amounts Recognized at Acquisition Date          
Business Combination, Recognized Identifiable Assets Acquired, Goodwill, and Liabilities Assumed, Net [Abstract]          
Cash and cash equivalents 93,460,000       93,460,000
Other current assets 703,501,000       703,501,000
Total current assets 796,961,000       796,961,000
Goodwill 4,034,868,000       4,034,868,000
Customer relationships intangible 1,184,800,000       1,184,800,000
Other long-term assets 88,729,000       88,729,000
Total assets acquired 6,105,358,000       6,105,358,000
Accounts payable 338,824,000       338,824,000
Pharmacy benefit management rebates payable 177,615,000       177,615,000
Accrued expenses and other current liabilities 194,672,000       194,672,000
Long-term debt 311,994,000       311,994,000
Other long-term liabilities 410,622,000       410,622,000
Total liabilities assumed 1,433,727,000       1,433,727,000
Non-controlling interest (1,610,000)       (1,610,000)
Net assets acquired 4,673,241,000       4,673,241,000
Catalyst | Employees Settlements          
Separate Tranactions and Preexisting Relationships [Abstract]          
Expenses and losses recognized       3,100,000  
Catalyst | Severence Cost          
Separate Tranactions and Preexisting Relationships [Abstract]          
Expenses and losses recognized       $ 10,400,000