Derivative Instruments and Balance Sheet Offsetting - Notional Amount and Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Derivative instruments
Notional Amount	$ 2,937,054	$ 2,210,925
Gross Derivative Assets	39,188	18,681
Gross Derivative Liabilities	20,407	12,357
Credit valuation adjustments for counterparty credit risk	383	809
Derivatives designated as hedging instruments:
Derivative instruments
Notional Amount	820,000	620,000
Gross Derivative Assets	17,618	5,899
Gross Derivative Liabilities	0	0
Derivatives designated as hedging instruments: | Interest rate swaps
Derivative instruments
Notional Amount	820,000	620,000
Gross Derivative Assets	17,618	5,899
Gross Derivative Liabilities	0	0
Derivatives not designated as hedging | Total customer-initiated and mortgage banking derivatives
Derivative instruments
Notional Amount	2,117,054	1,590,925
Gross Derivative Assets	21,570	12,782
Gross Derivative Liabilities	20,407	12,357
Derivatives not designated as hedging | Customer-initiated derivatives
Derivative instruments
Notional Amount	1,805,438	1,365,119
Gross Derivative Assets	18,275	9,376
Gross Derivative Liabilities	18,672	10,139
Derivatives not designated as hedging | Forward contracts related to mortgage loans to be delivered for sale
Derivative instruments
Notional Amount	150,508	115,996
Gross Derivative Assets	0	0
Gross Derivative Liabilities	425	34
Derivatives not designated as hedging | Interest rate lock commitments
Derivative instruments
Notional Amount	122,868	71,003
Gross Derivative Assets	1,985	1,222
Gross Derivative Liabilities	0	0
Derivatives not designated as hedging | Power Equity CD
Derivative instruments
Notional Amount	38,240	38,807
Gross Derivative Assets	1,310	2,184
Gross Derivative Liabilities	$ 1,310	$ 2,184