Derivative Instruments and Balance Sheet Offsetting - Notional Amount and Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative instruments
Notional Amount	$ 2,524,109	$ 2,210,925
Gross Derivative Assets	28,998	18,681
Gross Derivative Liabilities	13,960	12,357
Credit valuation adjustments for counterparty credit risk	416	809
Derivatives designated as hedging instruments:
Derivative instruments
Notional Amount	620,000	620,000
Gross Derivative Assets	14,104	5,899
Gross Derivative Liabilities	0	0
Derivatives designated as hedging instruments: | Interest rate swaps
Derivative instruments
Notional Amount	620,000	620,000
Gross Derivative Assets	14,104	5,899
Gross Derivative Liabilities	0	0
Derivatives not designated as hedging | Total customer-initiated and mortgage banking derivatives
Derivative instruments
Notional Amount	1,904,109	1,590,925
Gross Derivative Assets	14,894	12,782
Gross Derivative Liabilities	13,960	12,357
Derivatives not designated as hedging | Customer-initiated derivatives
Derivative instruments
Notional Amount	1,615,445	1,365,119
Gross Derivative Assets	11,619	9,376
Gross Derivative Liabilities	12,055	10,139
Derivatives not designated as hedging | Forward contracts related to mortgage loans to be delivered for sale
Derivative instruments
Notional Amount	137,066	115,996
Gross Derivative Assets	0	0
Gross Derivative Liabilities	404	34
Derivatives not designated as hedging | Interest rate lock commitments
Derivative instruments
Notional Amount	112,938	71,003
Gross Derivative Assets	1,774	1,222
Gross Derivative Liabilities	0	0
Derivatives not designated as hedging | Power Equity CD
Derivative instruments
Notional Amount	38,660	38,807
Gross Derivative Assets	1,501	2,184
Gross Derivative Liabilities	$ 1,501	$ 2,184