Securities - Contractual Maturities of Securities (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Available-for-sale, Amortized Cost
Due in one year or less	$ 1,788,000		$ 1,788,000
Due after one year through five years	106,284,000		106,284,000
Due after five years through ten years	236,000		236,000
Due after ten years	6,450,000		6,450,000
Total	114,758,000		114,758,000
Available-for-sale, Amortized Cost	697,245,000		697,245,000		$ 275,734,000
Available-for-sale, Fair Value
Due in one year or less	1,791,000		1,791,000
Due after one year through five years	106,186,000		106,186,000
Due after five years through ten years	243,000		243,000
Due after ten years	6,450,000		6,450,000
Total	114,670,000		114,670,000
Available-for-sale Securities, Total	694,259,000		694,259,000		274,619,000
Held-to-maturity, Amortized Cost
Held-to-maturity, Amortized Cost	13,730,000		13,730,000		35,505,000
Held-to-maturity, Fair Value
Held-to-maturity, Fair Value	13,701,000		13,701,000		34,844,000
Proceeds from sale of securities
Proceeds from Sale and Maturity of Marketable Securities		$ 0	349,400,000	$ 0
(Loss) gain on sale of available-for-sale securities, net	(332,000)		(38,000)
Gross realized gains			910,000
Gross realized losses			948,000
Impairment loss realized in consolidated statements of income	0		0
Mortgage-backed securities and collateralized mortgage obligations
Available-for-sale, Amortized Cost
Amortized cost	571,150,000		571,150,000
Available-for-sale, Fair Value
Fair value	568,478,000		568,478,000
Held-to-maturity, Amortized Cost
Debt maturities without single maturity date	13,730,000		13,730,000
Held-to-maturity, Fair Value
Debt maturities without single maturity date	13,701,000		13,701,000
CRA qualified investment fund
Available-for-sale, Amortized Cost
Amortized cost	11,337,000		11,337,000
Available-for-sale, Amortized Cost	11,337,000		11,337,000		11,183,000
Available-for-sale, Fair Value
Fair value	11,111,000		11,111,000
Available-for-sale Securities, Total	$ 11,111,000		$ 11,111,000		$ 10,913,000