Earnings per Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Basic earnings per share:
Net income (loss)	$ (25,851)	$ 77,891	$ (12,477)	$ 16,798	$ (9,968)	$ (262,146)	$ 70,739	$ 67,239	$ 56,361	$ (134,136)	$ 349,245
Less preferred stock dividends	(5,481)	(5,480)	(5,481)	(5,480)	(5,481)	(5,481)	(5,480)	(5,480)	(21,922)	(21,922)	(7,551)
Net income (loss) available to common shareholders	(31,332)	72,411	(17,958)	11,318	(15,449)	(267,627)	65,259	61,759	34,439	(156,058)	341,694
Weighted average shares	96,728,821	96,563,132	96,515,599	96,606,081	97,389,614	98,318,205	98,830,054	98,827,587	96,603,634	98,337,362	93,185,520
Basic earnings (loss) per share	$ (0.32)	$ 0.75	$ (0.19)	$ 0.12	$ (0.16)	$ (2.72)	$ 0.66	$ 0.62	$ 0.36	$ (1.59)	$ 3.67
Diluted earnings per share:
Net income (loss)	(25,851)	77,891	(12,477)	16,798	(9,968)	(262,146)	70,739	67,239	56,361	(134,136)	349,245
Less preferred stock dividends	(5,481)	(5,480)	(5,481)	(5,480)	(5,481)	(5,481)	(5,480)	(5,480)	(21,922)	(21,922)	(7,551)
Net income (loss) available to common shareholders									$ 34,439	$ (156,058)	$ 341,694
Weighted average shares	96,728,821	96,563,132	96,515,599	96,606,081	97,389,614	98,318,205	98,830,054	98,827,587	96,603,634	98,337,362	93,185,520
Diluted weighted average shares	96,728,821	96,563,132	96,515,599	96,606,081	97,389,614	98,318,205	98,830,054	98,827,587	96,603,634	98,337,362	93,185,520
Diluted earnings (loss) per share	$ (0.32)	$ 0.75	$ (0.19)	$ 0.12	$ (0.16)	$ (2.72)	$ 0.66	$ 0.62	$ 0.36	$ (1.59)	$ 3.67