Investment Securities - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized cost of available-for-sale securities	$ 1,019,247,000		$ 1,019,247,000		$ 719,607,000
Securities	987,076,000		987,076,000		710,570,000
Proceeds from sales/calls of securities available-for-sale	1,500,000	$ 129,300,000	11,513,000	$ 129,335,000
Gross gains on sale of available-for-sale derivatives			116,000	325,000
Gross loss on sale of available-for-sale derivatives				(2,400,000)
Fair value of securities sold under agreements to repurchase	129,500,000		129,500,000		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	264,184,000		264,184,000		239,618,000
Securities held-to-maturity, fair value	251,417,000		251,417,000		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	902,572,000		902,572,000		615,924,000
Securities	872,686,000		872,686,000		606,612,000
Fair value of securities sold under agreements to repurchase	95,400,000		95,400,000		78,200,000
Mortgage-backed securities and collateralized mortgage obligations
Schedule of Held-to-maturity Securities [Line Items]
Amortized cost	240,780,000		240,780,000		231,216,000
Securities held-to-maturity, fair value	$ 228,560,000		$ 228,560,000		$ 227,781,000