Condensed Consolidated Statements of Changes in Stockholders' Equity (Unaudited) - USD ($) $ in Thousands	Total	Common Shares	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Retained Earnings	Treasury Stock
Stockholders equity, beginning of period at Dec. 31, 2017	$ 1,940,436	$ 1,476	$ 1,127,137	$ (3,252)	$ 815,075	$ 0
Changes in Stockholders' Equity
Net income	338,835				338,835
Other comprehensive (loss) income	(44,197)			(44,197)
Issuance of management incentive shares		6	(6)
Stock-based compensation expense	11,269		11,269
Treasury stock acquired	(31,204)					(31,204)
Cancellation of treasury stock		(10)	(31,194)			31,204
Stockholders equity, end of period at Sep. 30, 2018	2,215,139	1,472	1,107,206	(47,449)	1,153,910	0
Stockholders equity, beginning of period at Jun. 30, 2018	2,103,571	1,472	1,103,448	(39,248)	1,037,899	0
Changes in Stockholders' Equity
Net income	116,011				116,011
Other comprehensive (loss) income	(8,201)			(8,201)
Stock-based compensation expense	3,837		3,837
Treasury stock acquired	(79)					(79)
Cancellation of treasury stock			(79)			79
Stockholders equity, end of period at Sep. 30, 2018	2,215,139	1,472	1,107,206	(47,449)	1,153,910	0
Stockholders equity, beginning of period at Dec. 31, 2018	2,365,717	1,472	1,110,800	(28,993)	1,282,438	0
Changes in Stockholders' Equity
Net income	408,755				408,755
Other comprehensive (loss) income	91,720			91,720
Dividends and dividend equivalents declared	(14,673)		139		(14,812)
Issuance of management incentive shares		7	(7)
Stock-based compensation expense	12,492		12,492
Treasury stock acquired	(8,841)					(8,841)
Cancellation of treasury stock		(3)	(8,838)			8,841
Stockholders equity, end of period at Sep. 30, 2019	2,855,170	1,476	1,114,586	62,727	1,676,381	0
Stockholders equity, beginning of period at Jun. 30, 2019	2,704,292	1,476	1,110,893	45,360	1,546,563	0
Changes in Stockholders' Equity
Net income	144,630				144,630
Other comprehensive (loss) income	17,367			17,367
Dividends and dividend equivalents declared	(14,673)		139		(14,812)
Stock-based compensation expense	4,170		4,170
Treasury stock acquired	(616)					(616)
Cancellation of treasury stock			(616)			616
Stockholders equity, end of period at Sep. 30, 2019	$ 2,855,170	$ 1,476	$ 1,114,586	$ 62,727	$ 1,676,381	$ 0