Net Income per Unit (Details) (USD $)
In Thousands, except Share data, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Schedule of net (loss) income per unit [Abstract]    
Net income - basic $ 45,585 $ 36,159
Additional allocation of equity in net income attributable to AB resulting from assumed dilutive effect of compensatory options 355 296
Net income - diluted $ 45,940 $ 36,455
Weighted average units outstanding - basic (in units) 100,548,000 96,114,000
Dilutive effect of compensatory options (in units) 1,098,000 1,077,000
Weighted average units outstanding - diluted (in units) 101,646,000 97,191,000
Basic net income per unit (in dollars per unit) $ 0.45 $ 0.38
Diluted net income per unit (in dollars per unit) $ 0.45 $ 0.38
Out-of-the-money options excluded from the diluted net income per unit (in units) 2,774,117 2,828,033