Fair Value - Recurring Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	$ 1,343,610	$ 1,332,658
Equity securities, at fair value	899	887
U.S. Treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	280	289
Obligations of U.S. Government sponsored entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	475,745	485,898
Obligations of U.S. states and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	87,778	85,440
U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	146,010	128,267
U.S. Government sponsored entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	631,328	630,558
Non-U.S. Government agencies or sponsored entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	19	31
U.S. corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	2,450	2,175
Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	0	0
Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	1,343,610	1,332,658
Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	0	0
Recurring | U.S. Treasuries | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	0	0
Recurring | U.S. Treasuries | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	280	289
Recurring | U.S. Treasuries | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	0	0
Recurring | Obligations of U.S. Government sponsored entities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	0	0
Recurring | Obligations of U.S. Government sponsored entities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	475,745	485,898
Recurring | Obligations of U.S. Government sponsored entities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	0	0
Recurring | Obligations of U.S. states and political subdivisions | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	0	0
Recurring | Obligations of U.S. states and political subdivisions | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	87,778	85,440
Recurring | Obligations of U.S. states and political subdivisions | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	0	0
Recurring | U.S. Government agencies | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	0	0
Recurring | U.S. Government agencies | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	146,010	128,267
Recurring | U.S. Government agencies | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	0	0
Recurring | U.S. Government sponsored entities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	0	0
Recurring | U.S. Government sponsored entities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	631,328	630,558
Recurring | U.S. Government sponsored entities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	0	0
Recurring | Non-U.S. Government agencies or sponsored entities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	0	0
Recurring | Non-U.S. Government agencies or sponsored entities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	19	31
Recurring | Non-U.S. Government agencies or sponsored entities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	0	0
Recurring | U.S. corporate debt securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	0	0
Recurring | U.S. corporate debt securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	2,450	2,175
Recurring | U.S. corporate debt securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	0	0
Recurring | Equity securities | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	0	0
Recurring | Equity securities | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	0	0
Recurring | Equity securities | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	899	887
Recurring | Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	1,343,610	1,332,658
Recurring | Fair Value | U.S. Treasuries
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	280	289
Recurring | Fair Value | Obligations of U.S. Government sponsored entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	475,745	485,898
Recurring | Fair Value | Obligations of U.S. states and political subdivisions
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	87,778	85,440
Recurring | Fair Value | U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	146,010	128,267
Recurring | Fair Value | U.S. Government sponsored entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	631,328	630,558
Recurring | Fair Value | Non-U.S. Government agencies or sponsored entities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	19	31
Recurring | Fair Value | U.S. corporate debt securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available for sale debt securities, fair value	2,450	2,175
Recurring | Fair Value | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity securities, at fair value	$ 899	$ 887