CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY (Unaudited) - USD ($)	Total	Common Stock	Additional Paid-in Capital	Retained Earnings	Accumulated Other Comp. Income (Loss), net
Balance at Dec. 31, 2017	$ 3,707,952,000	$ 77,740,000	$ 3,115,304,000	$ 519,144,000	$ (4,236,000)
Balance (in shares) at Dec. 31, 2017		77,740,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of employee common stock options and related tax benefits	1,616,000	$ 87,000	1,529,000
Exercise of employee common stock options and related tax benefits (in shares)		87,000
Common dividends paid	(10,974,000)			(10,974,000)
Issuance of restricted common shares, net of forfeitures	0	$ 106,000	(106,000)
Issuance of restricted common shares, net of forfeitures (in shares)		106,000
Restricted shares withheld for taxes and related tax benefit	(5,265,000)	$ (80,000)	(5,185,000)
Stock-based compensation expense	4,448,000		4,448,000
Restricted shares withheld for taxes (in shares)		(80,000)
Net income	83,510,000			83,510,000
Other comprehensive income (loss)	(31,984,000)				(31,984,000)
Balance at Mar. 31, 2018	3,749,303,000	$ 77,853,000	3,115,990,000	591,680,000	(36,220,000)
Balance (in shares) at Mar. 31, 2018		77,853,000
Balance at Dec. 31, 2018	3,965,940,000	$ 77,484,000	3,107,431,000	833,130,000	(52,105,000)
Balance (in shares) at Dec. 31, 2018		77,484,000
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Exercise of employee common stock options and related tax benefits	130,000	$ 5,000	125,000
Exercise of employee common stock options and related tax benefits (in shares)		5,000
Common dividends paid	(12,545,000)			(12,545,000)
Repurchase of common stock (in shares)		(543,000)
Repurchase of common stock	(30,049,000)	$ (543,000)	(29,506,000)
Issuance of restricted common shares, net of forfeitures	0	$ 180,000	(180,000)
Issuance of restricted common shares, net of forfeitures (in shares)		180,000
Restricted shares withheld for taxes and related tax benefit	(3,487,000)	$ (62,000)	(3,425,000)
Stock-based compensation expense	4,913,000		4,913,000
Restricted shares withheld for taxes (in shares)		(62,000)
Net income	93,960,000			93,960,000
Other comprehensive income (loss)	37,077,000				37,077,000
Balance at Mar. 31, 2019	$ 4,055,939,000	$ 77,064,000	$ 3,079,358,000	$ 914,545,000	$ (15,028,000)
Balance (in shares) at Mar. 31, 2019		77,064,000