Investments (Schedule of Amortized Cost and Estimated Fair Value of Debt Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 116,027
Amortized Cost, Due after one year through five years	308,477
Amortized Cost, Due after five years through ten years	123,518
Amortized Cost, Amortized Cost Due after ten years	40,265
Debt Maturities, Amortized Cost	604,999	$ 597,784
Fair Value, Due in one year or less	116,060
Fair Value, Due after one year through five years	308,829
Fair Value, Due after five years through ten years	124,168
Fair Value, Due after ten years	40,156
Debt securities	605,746	597,457
Collateralized Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Maturities, Amortized Cost	16,712
Debt securities	16,533	$ 59,469
Parent Company [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Maturities, Amortized Cost	150
Debt securities	$ 150