Statements of Shareholders’ Equity - USD ($) $ in Thousands	Total	Common Stock Voting Common Stock	Common Stock Non- Voting Common Stock	Additional Paid-In Capital	Treasury Stock	Retained Earnings (Accumulated Deficit)	Accumulated Other Comprehensive Income (Loss)
Balance at Dec. 31, 2012	$ 582,535	$ 6	$ 1	$ 504,527	$ (1,151)	$ 79,152	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment to liability recorded under Tax Receivable Agreement	2,336			2,336
Share-based compensation	5,689			5,689
Repurchase of common stock	(1,140)				(1,140)
Repurchase of common stock	0	1	(1)
Conversion of non-voting common stock to common stock	852			852
Proceeds from options exercised	1,927			1,927
Proceeds from options exercised	0
Net income	176,918					176,918
Balance at Dec. 31, 2013	769,117	7	0	515,331	(2,291)	256,070	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	8,797			8,797
Repurchase of common stock	(1,630)				(1,630)
Conversion of non-voting common stock to common stock	174			174
Proceeds from options exercised	1,871			1,871
Proceeds from options exercised	(718)						(718)
Net income	225,464					225,464
Balance at Dec. 31, 2014	1,003,075	7	0	526,173	(3,921)	481,534	(718)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Share-based compensation	9,222			9,222
Repurchase of common stock	(112,261)				(112,261)
Conversion of non-voting common stock to common stock	32			32
Proceeds from options exercised	8,850			8,850
Proceeds from options exercised	(828)						(828)
Net income	317,220					317,220
Balance at Dec. 31, 2015	$ 1,225,310	$ 7	$ 0	$ 544,277	$ (116,182)	$ 798,754	$ (1,546)