Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands									9 Months Ended
	Sep. 16, 2016	Sep. 15, 2016	Aug. 11, 2016	Jun. 15, 2016	Apr. 28, 2016	Mar. 15, 2016	Jan. 29, 2016	May 18, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Class of Stock [Line Items]
Preferred stock, par value (usd per share)									$ 1.0		$ 1.00
Net proceeds from issuance of preferred stock									$ 161,980	$ 55,569
Common stock, par value (usd per share)									$ 1.00		$ 1.00
Common stock, shares issued (shares)									28,074,477		27,432,061
Proceeds from issuance of common stock									$ 6,567	$ 730
Series F Preferred Stock
Class of Stock [Line Items]
Preferred stock, new issues (shares)	3,400,000
Preferred stock, par value (usd per share)	$ 1
Preferred stock share price (usd per share)	$ 25
Preferred stock, fixed dividend rate, percent	6.00%
Net proceeds from issuance of preferred stock	$ 82,300
Preferred stock. redemption price (usd per share)	$ 25
Series F Preferred Stock | London Interbank Offered Rate (LIBOR)
Class of Stock [Line Items]
Preferred stock, variable dividend rate, percent	4.762%
Common Stock
Class of Stock [Line Items]
Common stock, par value (usd per share)			$ 1
Common stock, maximum aggregate offering price			$ 50,000
Common stock, shares issued (shares)									219,386
Proceeds from issuance of common stock			$ 5,500
Series E Preferred Stock
Class of Stock [Line Items]
Preferred stock, new issues (shares)					2,300,000
Preferred stock, par value (usd per share)					$ 1.00
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. redemption price (usd per share)					$ 25.00
Preferred stock, dividend declared (usd per share)		$ 0.403125		$ 0.210521
Series E Preferred Stock | London Interbank Offered Rate (LIBOR)
Class of Stock [Line Items]
Preferred stock, variable dividend rate, percent					5.14%
Series D Preferred Stock
Class of Stock [Line Items]
Preferred stock, new issues (shares)							1,000,000
Preferred stock, par value (usd per share)							$ 1.00
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. redemption price (usd per share)							$ 25.00
Preferred stock, dividend declared (usd per share)		0.40625		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 C Preferred Stock
Class of Stock [Line Items]
Preferred stock, new issues (shares)								2,300,000
Preferred stock, par value (usd per share)								$ 1.00
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. redemption price (usd per share)								$ 25
Preferred stock, dividend declared (usd per share)		$ 0.4375		$ 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%