Investments (Fixed Maturity Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Available for sale securities, due in one year or less, amortized cost	$ 257,926
Available for sale securities, due after one year through five years, amortized cost	5,319,752
Available for sale securities, due after five years through ten years, amortized cost	9,837,190
Available for sale securities, due after ten years through twenty years, amortized cost	9,443,966
Available for sale securities, due after twenty years, amortized cost	8,618,803
Available for sale securities, securities with a single maturity date, amortized cost	33,477,637
Available for sale securities, amortized cost	44,353,741	$ 43,116,759
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available for sale securities, due in one year or less, fair value	263,997
Available for sale securities, due after one year through five years, fair value	5,332,148
Available for sale securities, due after five years through ten years, fair value	9,714,831
Available for sale securities, due after ten years through twenty years, fair value	9,881,974
Available for sale securities, due after twenty years, fair value	8,743,172
Available for sale securities, securities with a single maturity date, fair value	33,936,122
Available for sale securities, fair value	44,799,370	45,372,989
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Held for investment securities, due in one year or less, amortized cost	0
Held for investment securities, due after one year through five years, amortized cost	0
Held for investment securities, due after five years through ten years, amortized cost	0
Held for investment securities, due after ten years through twenty years, amortized cost	0
Held for investment securities, due after twenty years, amortized cost	77,155
Held for investment securities, securities with a single maturity date, amortized cost	77,155
Held for investment, amortized cost	77,155	77,041
Held-to-maturity Securities, Debt Maturities, Fair Value [Abstract]
Held for investment securities, due in one year or less, fair value	0
Held for investment securities, due after one year through five years, fair value	0
Held for investment securities, due after five years through ten years, fair value	0
Held for investment securities, due after ten years through twenty years, fair value	0
Held for investment securities, due after twenty years, fair value	68,108
Held for investment securities, securities with a single maturity date, fair value	68,108
Held for investment	68,108	$ 76,460
Residential Mortgage Backed Securities [Member]
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Available for sale securities, securities without a single maturity date, amortized cost	1,147,738
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available for sale securities, securities without a single maturity date, fair value	1,207,606
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Held for investment securities, securities without a single maturity date, amortized cost	0
Held-to-maturity Securities, Debt Maturities, Fair Value [Abstract]
Held for investment securities, securities without a single maturity date, fair value	0
Commercial Mortgage Backed Securities [Member]
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Available for sale securities, securities without a single maturity date, amortized cost	5,426,483
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available for sale securities, securities without a single maturity date, fair value	5,331,174
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Held for investment securities, securities without a single maturity date, amortized cost	0
Held-to-maturity Securities, Debt Maturities, Fair Value [Abstract]
Held for investment securities, securities without a single maturity date, fair value	0
Other Asset Backed Securities [Member]
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Available for sale securities, securities without a single maturity date, amortized cost	4,301,883
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available for sale securities, securities without a single maturity date, fair value	4,324,468
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Held for investment securities, securities without a single maturity date, amortized cost	0
Held-to-maturity Securities, Debt Maturities, Fair Value [Abstract]
Held for investment securities, securities without a single maturity date, fair value	$ 0