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	12 Months Ended			1 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended			1 Months Ended	12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Mar. 31, 2012 Series C Preferred Stock	Dec. 31, 2013 Series C Preferred Stock	Dec. 31, 2012 Series C Preferred Stock	Mar. 31, 2012 Series C Depositary Share	Oct. 31, 2006 Series C Depositary Share	Feb. 29, 2012 Series D Preferred Stock	Dec. 31, 2013 Series D Preferred Stock	Dec. 31, 2012 Series D Preferred Stock	Dec. 31, 2011 Series D Preferred Stock	Feb. 29, 2012 Series D Depositary Share	Dec. 31, 2013 Series D Depositary Share	Dec. 31, 2012 Series D Depositary Share	May 31, 2013 Series E Preferred Stock	Dec. 31, 2013 Series E Preferred Stock	Dec. 31, 2012 Series E Preferred Stock	Dec. 31, 2011 Series E Preferred Stock	May 31, 2013 Series E Depositary Share	Dec. 31, 2013 Series E Depositary Share	Dec. 31, 2012 Series E Depositary Share
Class of Stock [Line Items]
Dividend rate, percentage							7.375%				6.625%					6.625%		5.70%					5.70%
Depositary shares issued										3,680,000					11,500,000	11,500,000	11,500,000					11,500,000	11,500,000
Ratio of depositary share to underlying preferred stock										0.01					0.01							0.01
Stock redeemed during period, shares						3,680,000		3,680,000
Redemption price per share									$ 25.00			$ 2,500				$ 25.00			$ 2,500				$ 25.00
Aggregate redemption price per share									$ 25.0768229
Proceeds from issuance of Series D preferred stock												$ 0	$ 287,500	$ 0	$ 287,500				$ 287,500	$ 0	$ 0	$ 287,500
Stock issuance costs	$ 9,060	$ 10,393	$ 3,774	$ 2,265	$ 19,453	$ 3,098					$ 9,855							$ 9,856
Fixed annual dividend per share, depository shares																$ 1.65625							$ 1.425
Underwriters over allotment Of shares	1,050,000	1,200,000													1,500,000							1,500,000
Stated liquidation value per share																$ 25.00	$ 25						$ 25.00