CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common stock	Paid-in capital	Treasury stock	Accumulated other comprehensive income (loss)	Retained earnings
Balance, beginning of period at Dec. 31, 2017		$ 370,567	$ 1,850,582	$ 0	$ (43,801)	$ 977,178
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net common stock issued under share-based compensation plans		786	(8,917)
Reissuance of treasury stock, net under share-based compensation plans			0	0
Equity compensation			15,974
Repurchase of common stock				(100,059)
Other comprehensive income (loss), net of tax	$ (85,130)				(85,130)
Net income	512,351					512,351
Cash dividends						0
Balance, end of period at Sep. 30, 2018	3,489,531	371,353	1,857,639	(100,059)	(128,931)	1,489,529
Balance, beginning of period at Jun. 30, 2018		371,348	1,852,251	(100,059)	(117,294)	1,307,629
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net common stock issued under share-based compensation plans		5	(63)
Reissuance of treasury stock, net under share-based compensation plans			0	0
Equity compensation			5,451
Repurchase of common stock				0
Other comprehensive income (loss), net of tax	(11,637)				(11,637)
Net income	181,900					181,900
Cash dividends						0
Balance, end of period at Sep. 30, 2018	3,489,531	371,353	1,857,639	(100,059)	(128,931)	1,489,529
Balance, beginning of period at Dec. 31, 2018	3,581,891	371,353	1,862,536	(175,059)	(124,214)	1,647,275
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net common stock issued under share-based compensation plans		0	0
Reissuance of treasury stock, net under share-based compensation plans			(11,715)	5,989
Equity compensation			14,152
Repurchase of common stock				(94,126)
Other comprehensive income (loss), net of tax	187,996				187,996
Net income	496,653					496,653
Cash dividends						(21,223)
Balance, end of period at Sep. 30, 2019	4,159,617	371,353	1,864,973	(263,196)	63,782	2,122,705
Balance, beginning of period at Jun. 30, 2019		371,353	1,860,578	(194,070)	30,810	1,966,994
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net common stock issued under share-based compensation plans		0	0
Reissuance of treasury stock, net under share-based compensation plans			(133)	59
Equity compensation			4,528
Repurchase of common stock				(69,185)
Other comprehensive income (loss), net of tax	32,972				32,972
Net income	176,934					176,934
Cash dividends						(21,223)
Balance, end of period at Sep. 30, 2019	$ 4,159,617	$ 371,353	$ 1,864,973	$ (263,196)	$ 63,782	$ 2,122,705