Investment Securities - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of available-for-sale securities	$ 958,253,000		$ 958,253,000		$ 719,607,000
Securities	932,070,000		932,070,000		710,570,000
Proceeds from sales/calls of securities available-for-sale			9,966,000	$ 0
Gross gains on sale of available-for-sale derivatives			116,000
Fair value of securities sold under agreements to repurchase	138,400,000		138,400,000		204,300,000
Other-than-temporary impairment charge on available-for-sale securities			0	0
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	254,801,000		254,801,000		239,618,000
Securities held-to-maturity, fair value	244,239,000		244,239,000		236,125,000
Proceeds from sale of held-to-maturity securities	0	$ 0	0	0
Other-than-temporary impairment charge on held-to-maturity securities			0	$ 0
Mortgage-backed securities and collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of available-for-sale securities	840,040,000		840,040,000		615,924,000
Securities	815,532,000		815,532,000		606,612,000
Fair value of securities sold under agreements to repurchase	108,300,000		108,300,000		78,200,000
Mortgage-backed securities and collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	241,398,000		241,398,000		231,216,000
Securities held-to-maturity, fair value	$ 231,182,000		$ 231,182,000		$ 227,781,000