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		781	(8,854)
Reissuance of treasury stock, net under share-based compensation plans			0	0
Equity compensation			10,523
Repurchase of common stock				(100,059)
Other comprehensive income (loss), net of tax	$ (73,493)				(73,493)
Net income	330,451					330,451
Balance, end of period at Jun. 30, 2018	3,313,875	371,348	1,852,251	(100,059)	(117,294)	1,307,629
Balance, beginning of period at Mar. 31, 2018		371,348	1,847,000	0	(107,760)	1,120,815
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			0	0
Equity compensation			5,251
Repurchase of common stock				(100,059)
Other comprehensive income (loss), net of tax	(9,534)				(9,534)
Net income	186,814					186,814
Balance, end of period at Jun. 30, 2018	3,313,875	371,348	1,852,251	(100,059)	(117,294)	1,307,629
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			(11,582)	5,930
Equity compensation			9,624
Repurchase of common stock				(24,941)
Other comprehensive income (loss), net of tax	155,024				155,024
Net income	319,719					319,719
Balance, end of period at Jun. 30, 2019	4,035,665	371,353	1,860,578	(194,070)	30,810	1,966,994
Balance, beginning of period at Mar. 31, 2019		371,353	1,856,236	(169,129)	(41,493)	1,799,216
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			0	0
Equity compensation			4,342
Repurchase of common stock				(24,941)
Other comprehensive income (loss), net of tax	72,303				72,303
Net income	167,778					167,778
Balance, end of period at Jun. 30, 2019	$ 4,035,665	$ 371,353	$ 1,860,578	$ (194,070)	$ 30,810	$ 1,966,994