Derivative Instruments and Balance Sheet Offsetting (Notional Amount and Fair Value) (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Notional Amount	$ 2,210,925,000	$ 853,594,000
Gross Derivative Assets	18,681,000	8,215,000
Gross Derivative Liabilities	12,357,000	6,359,000
Credit valuation adjustments for counterparty credit risk	809,000	99,000
Designated as hedging
Derivative [Line Items]
Notional Amount	620,000,000	0
Gross Derivative Assets	5,899,000	0
Gross Derivative Liabilities	0	0
Designated as hedging | Interest rate swaps
Derivative [Line Items]
Notional Amount	620,000,000	0
Gross Derivative Assets	5,899,000	0
Gross Derivative Liabilities	0	0
Not designated as hedging
Derivative [Line Items]
Notional Amount	1,590,925,000	853,594,000
Gross Derivative Assets	12,782,000	8,215,000
Gross Derivative Liabilities	12,357,000	6,359,000
Not designated as hedging | Customer-initiated derivatives
Derivative [Line Items]
Notional Amount	1,365,119,000	600,598,000
Gross Derivative Assets	9,376,000	4,406,000
Gross Derivative Liabilities	10,139,000	4,141,000
Not designated as hedging | Forward contracts related to mortgage loans to be delivered for sale
Derivative [Line Items]
Notional Amount	115,996,000	140,155,000
Gross Derivative Assets	0	635,000
Gross Derivative Liabilities	34,000	0
Not designated as hedging | Interest rate lock commitments
Derivative [Line Items]
Notional Amount	71,003,000	76,034,000
Gross Derivative Assets	1,222,000	956,000
Gross Derivative Liabilities	0	0
Not designated as hedging | Power Equity CD
Derivative [Line Items]
Notional Amount	38,807,000	36,807,000
Gross Derivative Assets	2,184,000	2,218,000
Gross Derivative Liabilities	$ 2,184,000	$ 2,218,000