Offsetting Assets and Liabilities - Summary of amounts outstanding under master netting agreements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Offsetting [Abstract]
Repurchase and reverse repurchase agreements gross amounts of recognized assets	$ 400,000	$ 325,000	$ 320,000
Repurchase and reverse repurchase agreements gross amounts offset on balance sheet assets	(350,000)	(265,000)	(200,000)
Repurchase and reverse repurchase agreements net assets balance	50,000	60,000	120,000
Repurchase and reverse repurchase agreements gross amounts not off set in balance sheet assets of financial instruments
Repurchase and reverse repurchase agreements gross amounts not off set in balance sheet assets of collateral received
Repurchase and reverse repurchase agreements net assets	50,000	60,000	120,000
Securities lending transactions, gross amounts of recognized assets		50,000
Securities lending transactions, gross amounts offset on balance sheet assets		(50,000)
Securities lending transactions, net assets balance
Securities lending transactions, gross amounts not off set in balance sheet assets of financial instruments
Securities lending transactions, gross amounts not off set in balance sheet assets of collateral received
Securities lending transactions, assets
Derivative, gross amounts of recognized assets	9,017	658	223
Derivative, gross amounts offset on balance sheet
Derivatives asset net balance	9,017	658	223
Derivative, gross amounts not off set in balance sheet assets of financial instruments
Derivative, gross amounts not off set in balance sheet assets of collateral received
Derivative asset	9,017	658	223
Offsetting assets gross amounts of recognized assets	409,017	375,658	320,223
Offsetting assets gross amounts offset on balance sheet	(350,000)	(315,000)	(200,000)
Offsetting assets net asset balance	59,017	60,658	120,223
Offsetting assets gross amounts not offset in balance sheet of financial instruments
Offsetting assets gross amounts not offset in the balance sheet of collateral received
Offsetting assets net amount	59,017	60,658	120,223
Weighted average interest rate of reverse repurchase agreements assets	1.09%	1.18%	1.27%
Repurchase and reverse repurchase agreements gross amounts of recognized liabilities	350,000	265,000	200,000
Repurchase and reverse repurchase agreements gross amounts offset on balance sheet liabilities	(350,000)	(265,000)	(200,000)
Repurchase and reverse repurchase agreements net liabilities balance
Repurchase and reverse repurchase agreements gross amounts not off set in balance sheet liabilities of financial instruments
Repurchase and reverse repurchase agreements gross amounts not off set in balance sheet liabilities of collateral pledged
Repurchase and reverse repurchase agreements net liabilities
Securities lending transactions, gross amounts of recognized liabilities		50,000
Securities lending transactions, gross amounts offset on balance sheet		(50,000)
Securities lending transactions, net liabilities balance
Securities lending transactions, gross amounts not offset in balance sheet of financial instruments
Securities lending transactions, gross amounts not offset in the balance sheet of collateral pledged
Securities lending transactions, liability
Derivative liability, gross amounts of recognized liabilities	29,330	12,543	6,142
Derivative liability, gross amounts offset on balance sheet liabilities
Derivative liability, net liabilities balance	29,330	12,543	6,142
Derivative liability, gross amounts not off set in balance sheet liabilities of financial instruments
Derivative liability, gross amounts not off set in balance sheet liabilities of collateral pledged	18,198	11,493	5,405
Derivative liability	11,132	1,050	737
Offsetting liabilities gross amounts of recognized liabilities	379,330	327,543	206,142
Offsetting liabilities gross amounts offset on balance sheet	(350,000)	(315,000)	(200,000)
Offsetting liabilities, net liabilities balance	29,330	12,543	6,142
Offsetting liabilities gross amounts not offset in balance sheet of financial instruments
Offsetting liabilities gross amounts not offset in the balance sheet of collateral pledged	18,198	11,493	5,405
Offsetting liabilities net amount	$ 11,132	$ 1,050	$ 737
Weighted average interest rate of reverse repurchase agreements liabilities	0.25%	0.43%	0.41%