Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid-In Capital [Member]	Accumulated Other Comprehensive Income [Member]	Retained Earnings [Member]
Stockholders' equity at beginning of period at Dec. 31, 2014	$ 2,139,876	$ 76,062	$ 513,218	$ 721,401	$ 829,195
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	219,830				219,830
Other comprehensive income (loss)	(519,738)			(519,738)
Share-based compensation	9,976		9,976
Issuance of common stock	104,479	4,300	100,179
Issuance of common stock under compensation plans	7,986	944	7,042
Issuance of common stock to settle warrants that have reached their expiration	0	48	(48)
Dividends on common stock	(17,874)				(17,874)
Stockholders' equity at end of period at Dec. 31, 2015	1,944,535	81,354	630,367	201,663	1,031,151
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	83,243				83,243
Other comprehensive income (loss)	138,303			138,303
Share-based compensation	7,218		7,218
Issuance of common stock	134,662	5,590	129,072
Issuance of common stock under compensation plans	4,745	964	3,781
Issuance of common stock to settle warrants that have reached their expiration	(1)	93	(94)
Dividends on common stock	(21,110)				(21,110)
Stockholders' equity at end of period at Dec. 31, 2016	2,291,595	88,001	770,344	339,966	1,093,284
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	174,645				174,645
Other comprehensive income (loss)	384,633			384,633
Share-based compensation	6,464		6,464
Issuance of common stock under compensation plans	15,968	1,330	14,638
Dividends on common stock	(23,148)				(23,148)
Stockholders' equity at end of period at Dec. 31, 2017	$ 2,850,157	$ 89,331	$ 791,446	$ 724,599	$ 1,244,781