Preferred Stock (Details) (USD $) In Thousands, except Share data, unless otherwise specified	1 Months Ended		12 Months Ended			1 Months Ended	12 Months Ended					1 Months Ended		1 Months Ended	12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Mar. 31, 2012 Series C Preferred Stock [Member]	Dec. 31, 2012 Series C Preferred Stock [Member]	Dec. 31, 2012 Series C Depositary Share [Member]	Mar. 31, 2012 Series C Depositary Share [Member]	Dec. 31, 2011 Series C Depositary Share [Member]	Oct. 31, 2006 Series C Depositary Share [Member]	Feb. 29, 2012 Series D Preferred Stock [Member]	Dec. 31, 2012 Series D Preferred Stock [Member]	Feb. 29, 2012 Series D Depositary Share [Member]	Dec. 31, 2012 Series D Depositary Share [Member]	Dec. 31, 2011 Series D Depositary Share [Member]
Class of Stock [Line Items]
Dividend rate, percentage							7.375%					6.625%			6.625%
Depositary shares issued								0		3,680,000	3,680,000			11,500,000	11,500,000	0
Ratio of depositary share to underlying preferred stock											0.01			0.01
Stock redeemed during period, shares						3,680,000	3,680,000
Redemption price per share									$ 25				$ 2,500		$ 25
Aggregate redemption price per share									$ 25.0768229
Proceeds from issuance of preferred stock			$ 287,500	$ 0	$ 0									$ 287,500
Stock issuance costs	$ 9,060	$ 10,393	$ 2,265	$ 19,453	$ 1	$ 3,098						$ 9,855
Fixed annual dividend per share, depository shares															$ 1.65625
Underwriters over allotment Of shares	1,050,000	1,200,000												1,500,000
Stated liquidation value per share								$ 0		$ 25					$ 25	$ 0