Debt Securities Available for Sale - Narrative (Details)	3 Months Ended
Mar. 31, 2019 USD ($) security	Mar. 31, 2018 USD ($) security	Dec. 31, 2018 USD ($) security
Debt Securities, Available-for-sale [Line Items]
Amortized cost	$ 1,095,282,000		$ 1,049,932,000
Debt securities available for sale, at fair value	1,090,177,000		1,032,868,000
Proceeds from sales of equity securities	$ 0	$ 0
Number of matured debt securities \| security	1
Sale and maturity of debt securities, available-for-sale	$ 797,000	$ 9,700,000
Number of debt securities \| security		1
Available-for-sale securities, gross realized losses		$ 0
Gross gains on sale of available-for-sale derivatives		116,000
Fair value of securities pledged as collateral	$ 246,100,000		$ 232,700,000
Number of positions \| security	133		151
Other-than-temporary impairment charge on available-for-sale securities	$ 0	$ 0
Mortgage-backed securities and collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Amortized cost	970,978,000		$ 934,631,000
Debt securities available for sale, at fair value	$ 967,129,000		$ 920,007,000

Debt Securities, Available-for-sale, Number Of Debt Securities Called

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Debt Securities, Available-for-sale, Number Of Matured Debt Securities

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of restricted investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position.

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Amount of other-than-temporary impairment (OTTI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), recognized in earnings and other comprehensive loss (OCI).

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Amount of cash inflow from sale, maturity, prepayment and call of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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