CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - 6 months ended Jun. 30, 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	$ 5	4,153		4,158
Issuance of common stock upon vesting of restricted stock units and upon exercise of options (in shares)	599,984
Stock-based compensation		8,317		8,317
Cash dividends (declared $0.20 per share)			(20,796)	(20,796)
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			225,862	225,862
Balance at Jun. 30, 2018	$ 1,041	$ 2,265,393	$ 2,071,408	$ 4,337,842
Balance (in shares) at Jun. 30, 2018	104,065,045