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 (loss)	186,054				186,054
Other comprehensive income (loss)	(294,846)			(294,846)
Share-based compensation, including excess income tax benefits	7,567		7,567
Issuance of common stock	104,479	4,300	100,179
Issuance of common stock under compensation plans, including excess income tax benefits	6,779	829	5,950
Stockholders' equity at end of period at Sep. 30, 2015	2,149,909	81,191	626,914	426,555	1,015,249
Stockholders' equity at beginning 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 (loss)	(37,553)				(37,553)
Other comprehensive income (loss)	762,133			762,133
Share-based compensation, including excess income tax benefits	5,335		5,335
Issuance of common stock	134,662	5,590	129,072
Issuance of common stock under compensation plans, including excess income tax benefits	3,751	861	2,890
Issuance of common stock to settle warrants that have reached their expiration	(1)	93	(94)
Stockholders' equity at end of period at Sep. 30, 2016	$ 2,812,862	$ 87,898	$ 767,570	$ 963,796	$ 993,598