Securities (Details 3) (USD $)	3 Months Ended	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2013	Dec. 31, 2012
Amortized Cost
Due in one year or less	$ 44,895,000	$ 44,895,000
Due after one year through five years	110,294,000	110,294,000
Due after five years through ten years	87,047,000	87,047,000
Due after ten years	835,714,000	835,714,000
Total	1,077,950,000	1,077,950,000
Total amortized cost	1,274,079,000	1,274,079,000
Fair Value
Due in one year or less	45,076,000	45,076,000
Due after one year through five years	120,931,000	120,931,000
Due after five years through ten years	89,034,000	89,034,000
Due after ten years	797,103,000	797,103,000
Total	1,052,144,000	1,052,144,000
Fair Value	1,247,270,000	1,247,270,000
Realized net gain (loss) from trading securities portfolio	100,000	(2,600,000)
Carrying amount of securities pledged	928,700,000	928,700,000	635,200,000
Fair value of securities pledged	884,600,000	884,600,000	633,400,000
Deposit with various state insurance departments	9,400,000	9,400,000	9,300,000
Residential mortgage-backed securities
Amortized Cost
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	62,825,000	62,825,000
Fair Value
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	63,788,000	63,788,000
Collateralized mortgage obligations
Amortized Cost
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	132,563,000	132,563,000
Fair Value
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	130,534,000	130,534,000
Commercial mortgage-backed securities
Amortized Cost
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	741,000	741,000
Fair Value
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	$ 804,000	$ 804,000