Condensed Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Preferred Stock [Member]	Common Stock [Member]	Additional Paid-in Capital [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Treasury Stock [Member]
Stockholders' equity, beginning balance at Dec. 31, 2015	$ 4,055,552	$ 0	$ 130,218	$ 482,284	$ 231,947	$ 3,614,369	$ (403,266)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income (loss)	427,561				303,528	124,033
Common dividends declared ($0.14 in 2016 and $0.15 in 2017)	(16,979)					(16,979)
Acquisition of treasury stock	(85,089)						(85,089)
Stock-based compensation	6,935			388		(2,224)	8,771
Exercise of stock options	3,763					(4,020)	7,783
Stockholders' equity, ending balance at Mar. 31, 2016	4,391,743	0	130,218	482,672	535,475	3,715,179	(471,801)
Stockholders' equity, beginning balance at Dec. 31, 2016	4,566,861	0	127,218	490,421	577,574	3,890,798	(519,150)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Comprehensive income (loss)	278,832				145,291	133,541
Common dividends declared ($0.14 in 2016 and $0.15 in 2017)	(17,567)					(17,567)
Acquisition of treasury stock	(113,719)						(113,719)
Stock-based compensation	8,195			1,351		(606)	7,450
Exercise of stock options	22,225					(11,501)	33,726
Stockholders' equity, ending balance at Mar. 31, 2017	$ 4,744,827	$ 0	$ 127,218	$ 491,772	$ 722,865	$ 3,994,665	$ (591,693)