Statements of Shareholders’ Equity (USD $) In Thousands, unless otherwise specified	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, 2011	$ 466,706	$ 6	$ 1	$ 496,136	$ (129)	$ (29,308)	$ 0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Adjustment to liability recorded under Tax Receivable Agreement	1,497			1,497
Share-based compensation	4,327			4,327
Repurchase of common stock	(1,022)				(1,022)
Proceeds from options exercised	469			469
Excess tax benefits from share-based compensation	2,098			2,098
Unrealized gain (loss) from cash flow hedges, net of tax	0
Net income	108,460					108,460
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)
Conversion of non-voting common stock to common stock	0	1	(1)
Proceeds from options exercised	852			852
Excess tax benefits from share-based compensation	1,927			1,927
Unrealized gain (loss) from cash flow hedges, net of tax	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)
Proceeds from options exercised	174			174
Excess tax benefits from share-based compensation	1,871			1,871
Unrealized gain (loss) from cash flow hedges, net of tax	(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)