Balance Sheet Offsetting and Repurchase Agreements Accounted for as Secured Borrowings - Summary of amounts outstanding under master netting agreements (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Offsetting [Abstract]
Repurchase agreements / reverse repurchase agreements, gross amounts of recognized assets	$ 50,000	$ 100,000
Repurchase agreements / reverse repurchase agreements, gross amounts offset on balance sheet	(50,000)	(100,000)
Repurchase agreements / reverse repurchase agreements, net asset balance	0	0
Repurchase agreements / reverse repurchase agreements, gross amounts not offset in balance sheet - financial instruments	0	0
Repurchase agreements/ reverse repurchase agreements, gross amounts not offset in balance sheet - collateral received	0	0
Repurchase agreements / reverse repurchase agreements, net amount	0	0
Derivative, gross amounts of recognized assets	24,705	22,721
Derivative, gross amounts offset on balance sheet	0	0
Derivatives, net asset balance	24,705	22,721
Derivatives, gross amounts not offset in balance sheet - financial instruments	(973)	(1,490)
Derivatives, gross amounts not offset in balance sheet - collateral received	(8,029)	(6,369)
Derivatives asset, net amount	15,703	14,862
Offsetting assets, gross amounts of recognized assets	74,705	122,721
Offsetting assets, gross amounts offset on balance sheet	(50,000)	(100,000)
Offsetting assets, net asset balance	24,705	22,721
Offsetting assets, gross amounts not offset in balance sheet - financial instruments	(973)	(1,490)
Offsetting assets, gross amounts not offset in the balance sheet - collateral received	(8,029)	(6,369)
Offsetting assets, net amount	$ 15,703	$ 14,862
Weighted average interest rate of reverse repurchase agreements, assets (percent)	3.20%	1.95%
Repurchase agreements / reverse repurchase agreements, gross amounts of recognized liabilities	$ 50,000	$ 100,000
Repurchase agreements / reverse repurchase agreements, gross amounts offset on balance sheet	(50,000)	(100,000)
Repurchase agreements / reverse repurchase agreements, net liability balance	0	0
Repurchase agreements / reverse repurchase agreements, gross amounts not offset in balance sheet - financial instruments	0	0
Repurchase agreements / reverse repurchase agreements, gross amounts not offset in balance sheet - collateral pledged	0	0
Repurchase agreements / reverse repurchase agreements, net amount	0	0
Derivatives, gross amounts of recognized liabilities	26,433	25,376
Derivatives, gross amounts offset on balance sheet	0	0
Derivatives, net liability balance	26,433	25,376
Derivatives, gross amounts not offset in balance sheet - financial instruments	(973)	(1,490)
Derivatives, gross amounts not offset in balance sheet - collateral pledged	(16,126)	(17,190)
Derivatives, net amount	9,334	6,696
Offsetting liabilities, gross amounts of recognized liabilities	76,433	125,376
Offsetting liabilities, gross amounts offset on balance sheet	(50,000)	(100,000)
Offsetting liabilities, net liability balance	26,433	25,376
Offsetting liabilities gross, amounts not offset in balance sheet - financial instruments	(973)	(1,490)
Offsetting liabilities gross amounts not offset in the balance sheet - collateral pledged	(16,126)	(17,190)
Offsetting liabilities, net amount	$ 9,334	$ 6,696
Weighted average interest rate of reverse repurchase agreements, liabilities (percent)	2.45%	1.20%