Preferred Stock (Details) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended
	Feb. 28, 2017 $ / shares shares	Oct. 31, 2016 USD ($) $ / shares shares	May 31, 2013 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) shares	Dec. 31, 2015 USD ($)
Class of Stock [Line Items]
Stock Issuance Costs | $				$ 3,782	$ 4,266	$ 4,178
Ratio of depositary share to underlying preferred stock				0.01
Redemption price per share (in dollars per share) | $ / shares				$ 2,500.00
Stated liquidation value per share (in dollars per share) | $ / shares				$ 25.00
Preferred stock, shares redeemed (in shares) | shares	11,500,000
Series E Preferred Stock
Class of Stock [Line Items]
Dividend Rate (percent)			5.70%	5.70%	5.70%
Preferred shares issued (in shares) | shares			11,500,000	115,000	115,000
Gross Proceeds | $			$ 287,500
Stock Issuance Costs | $			$ 9,856
Dividend Per Depositary Share (in dollars per share) | $ / shares			$ 1.425000
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%		5.20%	5.20%
Preferred shares issued (in shares) | shares		13,800,000		138,000	138,000
Gross Proceeds | $		$ 345,000		$ 0	$ 345,000	0
Stock Issuance Costs | $		$ 10,897
Dividend Per Depositary Share (in dollars per share) | $ / shares		$ 1.30
Underwriters over allotment of shares (in shares) | shares				1,800,000
Series D Preferred Stock
Class of Stock [Line Items]
Dividend Rate (percent)	6.625%				6.625%
Preferred shares issued (in shares) | shares					115,000
Stated liquidation value per share (in dollars per share) | $ / shares	$ 25.00
Aggregate redemption price (in USD per share) | $ / shares	$ 25.3128472
Excess of redemption value over carrying value of Series D preferred shares redeemed | $				$ 9,855	$ 0	$ 0