Fair Value of Assets and Liabilities (Fair Value Assets Transfers between Level 1 and Level 2) (Details) - Available-for-sale Securities [Member] $ in Thousands	Dec. 31, 2017 USD ($)
Corporate Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	$ 596,809
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	88,674
Foreign Government Debt Securities [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair Value, Assets, Level 1 to Level 2 Transfers, Amount	317,640
Fair Value, Assets, Level 2 to Level 1 Transfers, Amount	$ 0