CONSOLIDATED STATEMENT OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Class A Common Stock	Class B Non-Voting Common Stock	Paid-in Capital	Retained Earnings	Total
Balance at Dec. 31, 2012	$ 991	$ 18	$ 2,198,501	$ 133,111	$ 2,332,621
Balance (in shares) at Dec. 31, 2012	99,417,998	1,829,339
Increase (Decrease) in Shareholders' Equity
Issuance of common stock upon exercise of options and vesting of restricted stock units (in shares)	1,023,521
Stock-based compensation expense			21,614		21,614
Common stock exchanged	$ 18	$ (18)
Common stock exchanged (in shares)	1,829,339	(1,829,339)
Cash dividends (declared $0.12 per share)				(10,663)	(10,663)
Tax benefits from stock based compensation arrangements			2,115		2,115
Tax withholdings related to vesting of restricted stock units			(12,664)		(12,664)
Tax withholdings related to vesting of restricted stock units (in shares)	(448,182)
Net income				190,411	190,411
Balance at Dec. 31, 2013	$ 1,009		2,209,566	312,859	2,523,434
Balance (in shares) at Dec. 31, 2013	101,822,676
Increase (Decrease) in Shareholders' Equity
Issuance of common stock upon exercise of options and vesting of restricted stock units	$ 1		943		944
Issuance of common stock upon exercise of options and vesting of restricted stock units (in shares)	1,028,654
Stock-based compensation expense			16,048		16,048
Cash dividends (declared $0.12 per share)				(13,284)	(13,284)
Tax benefits from stock based compensation arrangements			7,011		7,011
Tax withholdings related to vesting of restricted stock units			(18,089)		(18,089)
Tax withholdings related to vesting of restricted stock units (in shares)	(459,122)
Net income				255,998	255,998
Balance at Dec. 31, 2014	$ 1,010		2,215,479	555,573	2,772,062
Balance (in shares) at Dec. 31, 2014	102,392,208
Increase (Decrease) in Shareholders' Equity
Issuance of common stock upon exercise of options and vesting of restricted stock units			177		177
Issuance of common stock upon exercise of options and vesting of restricted stock units (in shares)	321,395
Stock-based compensation expense			17,022		17,022
Cash dividends (declared $0.12 per share)				(17,438)	(17,438)
Tax withholdings related to vesting of restricted stock units			(5,302)		(5,302)
Tax withholdings related to vesting of restricted stock units (in shares)	(130,934)
Net income				253,391	253,391
Balance at Dec. 31, 2015	$ 1,010		$ 2,227,376	$ 791,526	$ 3,019,912
Balance (in shares) at Dec. 31, 2015	102,582,669