Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands				3 Months Ended
	Mar. 15, 2016	Jan. 29, 2016	May. 18, 2015	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Class of Stock [Line Items]
Preferred stock, shares issued (shares)				3,300,000		2,300,000
Net proceeds from issuance of preferred stock				$ 24,108	$ 0
Preferred stock, par value (usd per share)				$ 1.0		$ 1.00
Preferred stock, liquidation preference (usd per share)				$ 25.00		$ 25.00
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, dividend declared (usd per share)	$ 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 C Preferred Stock
Class of Stock [Line Items]
Preferred stock, fixed dividend rate, percent				7.00%
Preferred stock, new issues (shares)			2,300,000
Preferred stock, par value (usd per share)			$ 1.00
Preferred stock, liquidation preference (usd per share)			$ 25.00
Preferred stock. redemption price (usd per share)				$ 25.00
Preferred stock, dividend declared (usd per share)	$ 0.4375
Series C Preferred Stock | London Interbank Offered Rate (LIBOR)
Class of Stock [Line Items]
Preferred stock, variable dividend rate, percent				5.30%