Preferred Stock (Details) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended
	Nov. 30, 2014 USD ($) shares	May. 31, 2013 USD ($) $ / shares shares	Feb. 29, 2012 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) shares	Dec. 31, 2013 USD ($)	Oct. 31, 2006
Class of Stock [Line Items]
Stock issuance costs | $	$ 8,488			$ 4,178	$ 10,683	$ 3,774
Underwriters over allotment of shares (in shares) | shares	712,500
Redemption price per share (in dollars per share) | $ / shares				$ 2,500.00
Stated liquidation value per share (in dollars per share) | $ / shares				$ 25.00
Series D Preferred Stock
Class of Stock [Line Items]
Dividend rate			6.625%	6.625%	6.625%
Depositary shares outstanding (in shares) | shares			11,500,000	115,000	115,000
Gross Proceeds | $			$ 287,500
Stock issuance costs | $			$ 9,855
Dividend per depositary share (in dollars per share) | $ / shares			$ 1.656250
Ratio of depositary share to underlying preferred stock							0.01
Underwriters over allotment of shares (in shares) | shares				1,500,000
Series E Preferred Stock
Class of Stock [Line Items]
Dividend rate		5.70%		5.70%	5.70%
Depositary shares outstanding (in shares) | shares		11,500,000		115,000	115,000
Gross Proceeds | $		$ 287,500		$ 0	$ 0	$ 287,500
Stock issuance costs | $		$ 9,856
Dividend per depositary share (in dollars per share) | $ / shares		$ 1.425000
Ratio of depositary share to underlying preferred stock				0.01
Underwriters over allotment of shares (in shares) | shares				1,500,000