Investments (Schedule of Amortized Cost and Estimated Fair Value of Debt Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Due in one year or less	$ 97,836
Amortized Cost, Due after one year through five years	272,190
Amortized Cost, Due after five years through ten years	114,461
Amortized Cost, Amortized Cost Due after ten years	53,524
Debt Maturities, Amortized Cost	597,784	$ 538,629
Fair Value, Due in one year or less	97,849
Fair Value, Due after one year through five years	272,168
Fair Value, Due after five years through ten years	115,248
Fair Value, Due after ten years	52,723
Debt securities	597,457	531,325
Collateralized Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Debt Maturities, Amortized Cost	59,773
Debt securities	$ 59,469	$ 59,383