Junior Subordinated Interest Deferrable Debentures (Details 2) (USD $) In Thousands, unless otherwise specified			3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Trust VI	Mar. 31, 2013 Trust VII	Mar. 31, 2013 Trust VIII	Mar. 31, 2013 Trust IX	Mar. 31, 2013 Trust X	Mar. 31, 2013 Trust XI	Mar. 31, 2013 Trust XII
Junior subordinated deferrable interest debentures, major types of business trusts
Junior subordinated deferrable interest debentures	$ 190,726	$ 190,726	$ 25,774	$ 10,310	$ 25,774	$ 41,238	$ 34,021	$ 32,990	$ 20,619
Interest rate (as a percent)			3.74%	3.55%	3.35%	1.93%	1.95%	1.93%	1.74%
Interest rate index			LIBOR	LIBOR	LIBOR	LIBOR	LIBOR	LIBOR	LIBOR
Spread on interest rate index (as a percent)			3.45%	3.25%	3.05%	1.62%	1.65%	1.62%	1.45%