Subsequent Events - Additional Information (Detail) (USD $)				1 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Single Issuer Trust Preferred Security [Member] Other Than Temporarily Impaired Securities [Member]	Oct. 31, 2013 Subsequent event [Member]	Oct. 25, 2013 Subsequent event [Member] Junior Subordinated Debentures [Member]
Subsequent Event [Line Items]
Amortized Cost	$ 934,704,000	$ 813,474,000	$ 41,800,000
Sale of net proceeds of debt securities				52,500,000
Realized gain on securities				10,700,000
Redemption of debentures					$ 131,300,000
Percentage of debentures issued					7.75%