Securities - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale securities, Continuous Unrealized Loss Position, Fair Value
Available-for-sale securities, Continuous Unrealized Loss Position, Fair Value, Less than 12 Months	$ 192,056
Available-for-sale securities, Continuous Unrealized Loss Position, Fair Value, 12 Months or Longer	183,667
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total	375,723
Available-for-sale securities, Continuous Unrealized Loss Position, Gross Unrealized Losses
Available-for-sale securities, Continuous Unrealized Loss Position, Gross Unrealized Losses, Less than 12 Months	911
Available-for-sale securities, Continuous Unrealized Loss Position, Gross Unrealized Losses, 12 Months or Longer	780
Available-for-sale securities, Continuous Unrealized Loss Position, Gross Unrealized Losses, Total	1,691
Treasury notes
Available-for-sale securities, Continuous Unrealized Loss Position, Fair Value
Available-for-sale securities, Continuous Unrealized Loss Position, Fair Value, Less than 12 Months	992
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total	992
Available-for-sale securities, Continuous Unrealized Loss Position, Gross Unrealized Losses
Available-for-sale securities, Continuous Unrealized Loss Position, Gross Unrealized Losses, Less than 12 Months	8
Available-for-sale securities, Continuous Unrealized Loss Position, Gross Unrealized Losses, Total	8
Federal agencies
Available-for-sale securities, Continuous Unrealized Loss Position, Fair Value
Available-for-sale securities, Continuous Unrealized Loss Position, Fair Value, Less than 12 Months	191,064	$ 275,143
Available-for-sale securities, Continuous Unrealized Loss Position, Fair Value, 12 Months or Longer	183,667
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total	374,731	275,143
Available-for-sale securities, Continuous Unrealized Loss Position, Gross Unrealized Losses
Available-for-sale securities, Continuous Unrealized Loss Position, Gross Unrealized Losses, Less than 12 Months	903	1,057
Available-for-sale securities, Continuous Unrealized Loss Position, Gross Unrealized Losses, 12 Months or Longer	780
Available-for-sale securities, Continuous Unrealized Loss Position, Gross Unrealized Losses, Total	$ 1,683	$ 1,057