Securities - Continuous Unrealized Loss Position (Details) - USD ($) $ in Thousands	Sep. 30, 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	$ 276,567
Available-for-sale securities, Continuous Unrealized Loss Position, Fair Value, 12 Months or Longer	112,946
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total	389,513
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	514
Available-for-sale securities, Continuous Unrealized Loss Position, Gross Unrealized Losses, 12 Months or Longer	279
Available-for-sale securities, Continuous Unrealized Loss Position, Gross Unrealized Losses, Total	793
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	997
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total	997
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	3
Available-for-sale securities, Continuous Unrealized Loss Position, Gross Unrealized Losses, Total	3
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	275,570	$ 275,143
Available-for-sale securities, Continuous Unrealized Loss Position, Fair Value, 12 Months or Longer	112,946
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value, Total	388,516	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	511	1,057
Available-for-sale securities, Continuous Unrealized Loss Position, Gross Unrealized Losses, 12 Months or Longer	279
Available-for-sale securities, Continuous Unrealized Loss Position, Gross Unrealized Losses, Total	$ 790	$ 1,057