Derivative Instruments - Non-hedge Derivatives (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Oct. 24, 2018	Oct. 23, 2018	Dec. 31, 2017
Derivative [Line Items]
Notional Amount					$ 101,000,000	$ 176,800,000
Not Designated as Hedging Instrument
Derivative [Line Items]
Description of Location of Gain (Loss) on Interest Rate Derivative on Income Statement			Other noninterest income
Derivative, Gain (Loss) on Derivative, Net	$ (8,000)	$ 7,000	$ (42,000)	$ 20,000
Notional Amount	1,945,632,000		1,945,632,000				$ 1,497,250,000
Estimated Fair Value	(137,000)		(137,000)				(95,000)
Not Designated as Hedging Instrument | Pay fixed / receive variable swaps
Derivative [Line Items]
Notional Amount	972,816,000		972,816,000				748,625,000
Estimated Fair Value	24,592,000		24,592,000				13,771,000
Not Designated as Hedging Instrument | Pay variable / receive fixed swaps
Derivative [Line Items]
Notional Amount	972,816,000		972,816,000				748,625,000
Estimated Fair Value	(28,436,000)		(28,436,000)				(13,866,000)
Not Designated as Hedging Instrument | Cash Settlements [Member]
Derivative [Line Items]
Notional Amount	0		0				0
Estimated Fair Value	$ 3,707,000		$ 3,707,000				$ 0