Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands						6 Months Ended
	Jun. 15, 2016	Apr. 28, 2016	Mar. 15, 2016	Jan. 29, 2016	May 18, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Class of Stock [Line Items]
Preferred stock, shares issued (shares)						5,600,000		2,300,000
Net proceeds from issuance of preferred stock						$ 79,701	$ 55,569
Preferred stock, par value (usd per share)						$ 1.0		$ 1.00
Preferred stock, liquidation preference (usd per share)						$ 25.00		$ 25.00
Series C Preferred Stock
Class of Stock [Line Items]
Preferred stock share price (usd per share)					$ 25.00
Preferred stock, fixed dividend rate, percent					7.00%
Net proceeds from issuance of preferred stock					$ 55,600
Preferred stock, new issues (shares)					2,300,000
Preferred stock, par value (usd per share)					$ 1.00
Preferred stock. redemption price (usd per share)					$ 25.00
Preferred stock, dividend declared (usd per share)	$ 0.4375		$ 0.4375
Series C Preferred Stock | London Interbank Offered Rate (LIBOR)
Class of Stock [Line Items]
Preferred stock, variable dividend rate, percent					5.30%
Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares issued (shares)				1,000,000
Preferred stock share price (usd per share)				$ 25.00
Preferred stock, fixed dividend rate, percent				6.50%
Net proceeds from issuance of preferred stock				$ 24,100
Preferred stock, par value (usd per share)				$ 1.00
Preferred stock. redemption price (usd per share)				$ 25.00
Preferred stock, dividend declared (usd per share)	0.40625		$ 0.2076
Series D Preferred Stock | London Interbank Offered Rate (LIBOR)
Class of Stock [Line Items]
Preferred stock, variable dividend rate, percent				5.09%
Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock, shares issued (shares)		2,300,000
Preferred stock share price (usd per share)		$ 25.00
Preferred stock, fixed dividend rate, percent		6.45%
Net proceeds from issuance of preferred stock		$ 55,600
Preferred stock, par value (usd per share)		$ 1.00
Preferred stock. redemption price (usd per share)		$ 25.00
Preferred stock, dividend declared (usd per share)	$ 0.210521
Series E Preferred Stock | London Interbank Offered Rate (LIBOR)
Class of Stock [Line Items]
Preferred stock, variable dividend rate, percent		5.14%