CONSOLIDATED BALANCE SHEETS (UNAUDITED) [Parenthetical] - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Held-to-maturity securities, fair value (dollars in thousands)	$ 595,815	$ 614,272
Common stock, par value (in dollars per share)	$ 250.00	$ 250.00
Common stock, shares authorized	25,000	25,000
Common stock, shares issued	23,661	23,661
Common stock, shares outstanding	23,661	23,661
Class A Convertible Adjustable Rate Preferred Stock [Member]
Preferred stock, par value (in dollars per share)	$ 20.00	$ 20.00
Preferred stock, shares authorized	750,000	750,000
Preferred stock, shares issued	641,082	641,082
Preferred stock, shares outstanding	641,082	641,082
Class B Adjustable Rate Preferred Stock [Member]
Preferred stock, par value (in dollars per share)	$ 1.50	$ 1.50
Preferred stock, shares authorized	200,000	200,000
Preferred stock, shares issued	160,505	160,505
Preferred stock, shares outstanding	160,505	160,505
Class C Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Preferred stock, par value (in dollars per share)	$ 1.00	$ 1.00
Preferred stock, shares authorized	295,400	295,400
Preferred stock, shares issued	295,400	295,400
Preferred stock, shares outstanding	295,400	295,400
Preferred stock, aggregate liquidation preference (dollars in thousands)	$ 428,979	$ 401,059
Class D Fixed Rate Cumulative Perpetual Preferred Stock [Member]
Preferred stock, par value (in dollars per share)	$ 1.00	$ 1.00
Preferred stock, shares authorized	14,770	14,770
Preferred stock, shares issued	14,770	14,770
Preferred stock, shares outstanding	14,770	14,770
Preferred stock, aggregate liquidation preference (dollars in thousands)	$ 25,887	$ 24,205