Preferred Stock (Details) $ / shares in Units, $ in Thousands		1 Months Ended								12 Months Ended
		Jan. 31, 2017	Oct. 31, 2016 USD ($) $ / shares shares		Nov. 30, 2014 USD ($) shares	May 31, 2013 USD ($) $ / shares shares		Feb. 29, 2012 USD ($) $ / shares shares		Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) shares	Dec. 31, 2014 USD ($)	Feb. 23, 2017 $ / shares
Class of Stock [Line Items]
Stock issuance costs | $					$ 8,488					$ 4,266	$ 4,178	$ 10,683
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 (percent)								6.625%	[1]	6.625%	6.625%
Depositary shares issued (in shares) | shares								11,500,000	[2]	115,000	115,000
Gross Proceeds | $								$ 287,500
Stock issuance costs | $	[3]							$ 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 (percent)						5.70%	[1]			5.70%	5.70%
Depositary shares issued (in shares) | shares						11,500,000	[2]			115,000	115,000
Gross Proceeds | $						$ 287,500
Stock issuance costs | $	[3]					$ 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
Series F Preferred Stock
Class of Stock [Line Items]
Dividend rate (percent)			5.20%	[1]						5.20%	0.00%
Depositary shares issued (in shares) | shares			13,800,000	[2]						138,000	0
Gross Proceeds | $			$ 345,000							$ 345,000	$ 0	$ 0
Stock issuance costs | $	[3]		$ 10,897
Dividend per depositary share (in dollars per share) | $ / shares			$ 1.30
Ratio of depositary share to underlying preferred stock										0.01
Underwriters over allotment of shares (in shares) | shares										1,800,000
Scenario, Forecast | Series D Preferred Stock
Class of Stock [Line Items]
Stated liquidation value per share (in dollars per share) | $ / shares													$ 25.00
Aggregate redemption price (in USD per share) | $ / shares													$ 25.3128472
Subsequent Event | Series D Preferred Stock
Class of Stock [Line Items]
Dividend rate (percent)	[1]	6.625%

[1] Holders are entitled to receive, when and as authorized by the Board of Directors, cumulative preferential cash dividends.

[2] Representing 1/100th of a preferred share. Series D and E issuances each included 1,500,000 depositary shares in connection with the underwriters' over-allotment.

[3] Consisting primarily of underwriting commissions and fees, rating agency fees, legal and accounting fees and printing expenses.