Balance Sheet Offsetting - Summary of amounts outstanding under master netting agreements (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Offsetting [Abstract]
Repurchase agreements / reverse repurchase agreements, gross amounts of recognized assets	$ 300,000	$ 400,000
Repurchase agreements / reverse repurchase agreements, gross amounts offset on balance sheet assets	(300,000)	(400,000)
Repurchase agreements / reverse repurchase agreements, net assets balance
Repurchase agreements / reverse repurchase agreements, gross amounts not off set in balance sheet assets of financial instruments
Repurchase agreements / reverse repurchase agreements, gross amounts not off set in balance sheet assets of collateral received
Repurchase agreements / reverse repurchase agreements, net amount
Derivatives, gross amounts of recognized assets	26,880	20,082
Derivatives, gross amounts offset on the balance sheet
Derivatives, net asset balance	26,880	20,082
Derivatives, gross amounts not off set in balance sheet assets of financial instruments	(1,779)	(519)
Derivative, gross amounts not off set in balance sheet assets of collateral received	(4,760)	(3,729)
Derivatives, net amount	20,341	15,834
Offsetting assets gross amounts offset on balance sheet	326,880	420,082
Offsetting assets, gross amounts offset on balance sheet	(300,000)	(400,000)
Offsetting assets, net asset balance	26,880	20,082
Offsetting assets, gross amounts not offset in balance sheet of financial instruments	(1,779)	(519)
Offsetting assets, gross amounts not offset in the balance sheet of collateral received	(4,760)	(3,729)
Offsetting assets, net amount	$ 20,341	$ 15,834
Weighted average interest rate of reverse repurchase agreements, assets	1.43%	1.34%
Repurchase agreements / reverse repurchase agreements, gross amounts of recognized liabilities	$ 300,000	$ 400,000
Repurchase agreements / reverse repurchase agreements, gross amounts offset on balance sheet liabilities	(300,000)	(400,000)
Repurchase agreements / reverse repurchase agreements, net liabilities balance
Repurchase agreements / reverse repurchase agreements, gross amounts not off set in balance sheet liabilities of financial instruments
Repurchase agreements / reverse repurchase agreements, gross amounts not off set in balance sheet liabilities of collateral pledged
Repurchase agreements / reverse repurchase agreements, net amount
Derivatives, gross amounts of recognized liabilities	34,930	28,825
Derivatives, gross amounts offset on balance sheet liabilities
Derivatives, net liabilities balance	34,930	28,825
Derivatives, gross amounts not off set in balance sheet liabilities of financial instruments	(1,779)	(519)
Derivatives, gross amounts not off set in balance sheet liabilities of collateral pledged	(33,182)	(30,917)
Derivatives, net amount
Offsetting liabilities, gross amounts of recognized liabilities	334,930	428,825
Offsetting liabilities, gross amounts offset on balance sheet	(300,000)	(400,000)
Offsetting liabilities, net liabilities balance	34,930	28,825
Offsetting liabilities, gross amounts not offset in balance sheet of financial instruments	(1,779)	(519)
Offsetting liabilities, gross amounts not offset in the balance sheet of collateral pledged	(33,182)	(30,917)
Offsetting liabilities, net amount
Weighted average interest rate of reverse repurchase agreements, liabilities	0.58%	0.50%