Derivative Instruments and Balance Sheet Offsetting - Notional Amount and Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivative instruments
Notional Amount	$ 3,086,518	$ 2,210,925
Gross Derivative Assets	45,561	18,681
Gross Derivative Liabilities	22,966	12,357
Credit valuation adjustments for counterparty credit risk	272	809
Derivatives designated as hedging instruments:
Derivative instruments
Notional Amount	820,000	620,000
Gross Derivative Assets	21,067	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	21,067	5,899
Gross Derivative Liabilities	0	0
Derivatives not designated as hedging | Total customer-initiated and mortgage banking derivatives
Derivative instruments
Notional Amount	2,266,518	1,590,925
Gross Derivative Assets	24,494	12,782
Gross Derivative Liabilities	22,966	12,357
Derivatives not designated as hedging | Customer-initiated derivatives
Derivative instruments
Notional Amount	1,960,013	1,365,119
Gross Derivative Assets	21,390	9,376
Gross Derivative Liabilities	21,703	10,139
Derivatives not designated as hedging | Foreign exchange forwards
Derivative instruments
Notional Amount	934	0
Gross Derivative Assets	3	0
Gross Derivative Liabilities	3	0
Derivatives not designated as hedging | Forward contracts related to mortgage loans to be delivered for sale
Derivative instruments
Notional Amount	169,554	115,996
Gross Derivative Assets	760	0
Gross Derivative Liabilities	0	34
Derivatives not designated as hedging | Interest rate lock commitments
Derivative instruments
Notional Amount	98,516	71,003
Gross Derivative Assets	1,081	1,222
Gross Derivative Liabilities	0	0
Derivatives not designated as hedging | Power Equity CD
Derivative instruments
Notional Amount	37,501	38,807
Gross Derivative Assets	1,260	2,184
Gross Derivative Liabilities	$ 1,260	$ 2,184