Derivatives - Derivative Financial Instruments Not Designated as hedging Instruments (Details) - Not Designated as Hedging Instrument - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Derivative Gains, notional amount	$ 634,758	$ 610,077
Derivative Gains, fair value	13,350	8,766
Derivative Losses, notional amount	651,353	636,681
Derivative Losses, fair value	13,338	8,785
Commercial loan level interest rate swap agreements
Derivatives, Fair Value [Line Items]
Derivative Gains, notional amount	597,239	582,388
Derivative Gains, fair value	13,313	8,741
Derivative Losses, notional amount	597,239	582,388
Derivative Losses, fair value	13,313	8,741
Other contracts
Derivatives, Fair Value [Line Items]
Derivative Gains, notional amount	37,519	27,689
Derivative Gains, fair value	37	25
Derivative Losses, notional amount	54,114	54,293
Derivative Losses, fair value	$ 25	$ 44