CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - 3 months ended Mar. 31, 2018 - USD ($) $ in Thousands	Class A Common Stock	Paid-in Capital	Retained Earnings	Total
Balance at Dec. 31, 2017	$ 1,036	$ 2,260,064	$ 1,866,342	$ 4,127,442
Balance (in shares) at Dec. 31, 2017	103,621,629
Increase (Decrease) in Shareholders' Equity
Issuance of common stock upon vesting of restricted stock units and upon exercise of options	$ 4	2,632		2,636
Issuance of common stock upon vesting of restricted stock units and upon exercise of options (in shares)	514,773
Stock-based compensation		3,432		3,432
Cash dividends (declared $0.10 per share)			(10,397)	(10,397)
Tax withholding related to vesting of restricted stock units and exercise of stock options		(7,141)		(7,141)
Tax withholding related to vesting of restricted stock units and exercise of stock options (in shares)	(156,568)
Net income			110,651	110,651
Balance at Mar. 31, 2018	$ 1,040	$ 2,258,987	$ 1,966,596	$ 4,226,623
Balance (in shares) at Mar. 31, 2018	103,979,834