Preferred Stock (Details) $ / shares in Units, $ in Thousands	1 Months Ended			12 Months Ended
	Feb. 28, 2017 USD ($) $ / shares shares	Oct. 31, 2016 USD ($) $ / shares shares	May 31, 2013 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	Dec. 31, 2016 USD ($)
Class of Stock [Line Items]
Stock issuance costs | $				$ 3,947	$ 3,782	$ 4,266
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
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
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		5.20%		5.20%	5.20%
Depositary shares outstanding (in shares) | shares		13,800,000		138,000	138,000
Gross proceeds | $		$ 345,000		$ 0	$ 0	345,000
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	6.625%
Stated liquidation value per share (in dollars per share) | $ / shares	$ 25.00
Preferred stock, shares redeemed (in shares) | shares	11,500,000				11,500,000
Aggregate redemption price (in dollars per share) | $ / shares	$ 25.3128472
Excess of redemption value over carrying value of preferred shares redeemed | $	$ 9,855			$ 0	$ 9,855	$ 0