Income Taxes (Components of Income Tax) (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Current tax expense      
Current tax expense $ 192,233,000 $ 147,739,000 $ 107,241,000
Deferred tax expense (benefit)      
Deferred tax expense (58,047,000) (44,336,000) (37,925,000)
Income tax expense 134,186,000 103,403,000 69,316,000
Income from U.S. operations before income taxes 428,700,000 319,800,000 158,700,000
United States      
Current tax expense      
Current tax expense 192,021,000 146,484,000 94,834,000
Deferred tax expense (benefit)      
Deferred tax expense (57,889,000) (44,180,000) (38,441,000)
Foreign      
Current tax expense      
Current tax expense 212,000 1,255,000 12,407,000
Deferred tax expense (benefit)      
Deferred tax expense (158,000) (156,000) 516,000
Income from Canadian operations before income taxes $ 79,200,000 $ 82,300,000 $ 31,600,000