Fair Value of Financial Instruments (Assets and Liabilities Measured on Recurring Basis) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	$ 3,084,789	$ 3,041,131
Equity securities	18,173	18,478
Derivatives, index options	116,356	194,731
Total assets	3,219,318	3,254,340
Policyholder account balances	134,293	211,159
Other liabilities	13,209	15,242
Total liabilities	$ 147,502	$ 226,401
Percent of total (in percentage)	100.00%	100.00%
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	$ 0	$ 0
Equity securities	18,173	18,478
Derivatives, index options	0	0
Total assets	18,173	18,478
Policyholder account balances	0	0
Other liabilities	0	0
Total liabilities	$ 0	$ 0
Percent of total (in percentage)	0.60%	0.60%
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	$ 3,084,789	$ 3,041,131
Equity securities	0	0
Derivatives, index options	0	0
Total assets	3,084,789	3,041,131
Policyholder account balances	0	0
Other liabilities	0	0
Total liabilities	$ 0	$ 0
Percent of total (in percentage)	95.80%	93.40%
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	$ 0	$ 0
Equity securities	0	0
Derivatives, index options	116,356	194,731
Total assets	116,356	194,731
Policyholder account balances	134,293	211,159
Other liabilities	13,209	15,242
Total liabilities	$ 147,502	$ 226,401
Percent of total (in percentage)	3.60%	6.00%
Priced by Third-Party Vendors
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	$ 3,084,789	$ 3,041,131
Equity securities	18,173	18,478
Derivatives, index options	116,356	194,731
Priced by Third-Party Vendors | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Equity securities	18,173	18,478
Derivatives, index options	0	0
Priced by Third-Party Vendors | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	3,084,789	3,041,131
Equity securities	0	0
Derivatives, index options	0	0
Priced by Third-Party Vendors | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Equity securities	0	0
Derivatives, index options	116,356	194,731
Priced Internally
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Equity securities	0	0
Derivatives, index options	0	0
Priced Internally | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Equity securities	0	0
Derivatives, index options	0	0
Priced Internally | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Equity securities	0	0
Derivatives, index options	0	0
Priced Internally | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities available for sale	0	0
Equity securities	0	0
Derivatives, index options	$ 0	$ 0