Fair Value - Fair Value Measurements for Items Measured at Fair Value (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities		$ 982,958	$ 949,460
Loans held for sale		20,887	24,306
Other real estate owned		10,288	7,640
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities		982,958	949,460	[1]
Loans held for sale	[2]	20,887	24,306
Equity securities	[3]	6,242	6,344
Fair Value, Measurements, Recurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities		100	100	[1]
Loans held for sale	[2]	0	0
Equity securities	[3]	6,242	6,344
Fair Value, Measurements, Recurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities		982,858	949,360	[1]
Loans held for sale	[2]	20,887	24,306
Equity securities	[3]	0	0
Fair Value, Measurements, Recurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities		0	0	[1]
Loans held for sale	[2]	0	0
Equity securities	[3]	0	0
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	[4]	7,139	4,192
Other real estate owned	[5]	10,288	7,640
Fair Value, Measurements, Nonrecurring [Member] | Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	[4]	0	0
Other real estate owned	[5]	0	0
Fair Value, Measurements, Nonrecurring [Member] | Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	[4]	6,607	3,454
Other real estate owned	[5]	3,826	60
Fair Value, Measurements, Nonrecurring [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans	[4]	532	738
Other real estate owned	[5]	$ 6,462	$ 7,580

[1] See Note D for further detail of fair value of individual investment categories.

[2] Recurring fair value basis determined using observable market data.

[3] Prior to adoption of ASU 2016-01 on January 1, 2018, an investment in shares of a mutual fund that invests primarily in CRA-qualified debt securities was classified as an available for sale security. Beginning in 2018, this security is reported at fair value in Other Assets. Fair value is determined based on market quotations.

[4] See Note F. Nonrecurring fair value adjustments to loans identified as impaired reflect full or partial write-downs that are based on the loan’s observable market price or current appraised value of the collateral in accordance with ASC 310.

[5] Fair value is measured on a nonrecurring basis in accordance with ASC 360.