CONSOLIDATED STATEMENTS OF SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common stock	Paid-in capital	Treasury stock	Accumulated other comprehensive (loss) income	Retained earnings
Balance, beginning of period at Dec. 31, 2015		$ 340,097	$ 1,670,238	$ (3,362)	$ (60,880)	$ 290,047
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net common stock issued under share-based compensation plans		985	(5,989)
Issuance of common stock		18,313	113,146
Tax benefit from share-based compensation			100
Equity compensation			8,753
Reacquisition of convertible junior subordinated debentures-equity component			(6,337)
Purchases of common stock	$ (108,100)			(108,097)
Reissuance of treasury stock, net				0		0
Other comprehensive income, net of tax	91,035				91,035
Net income	235,030					235,030
Balance, end of period at Sep. 30, 2016	2,583,079	359,395	1,779,911	(111,459)	30,155	525,077
Balance, beginning of period at Dec. 31, 2016	2,548,842	359,400	1,782,337	(150,359)	(75,100)	632,717
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net common stock issued under share-based compensation plans		776	(7,558)
Issuance of common stock		10,386	60,903
Tax benefit from share-based compensation			0
Equity compensation			10,578
Reacquisition of convertible junior subordinated debentures-equity component			0
Purchases of common stock				0
Reissuance of treasury stock, net				150,359		(21,740)
Other comprehensive income, net of tax	49,003				49,003
Net income	328,447					328,447
Balance, end of period at Sep. 30, 2017	$ 3,130,149	$ 370,562	$ 1,846,260	$ 0	$ (26,097)	$ 939,424