Investments (Fixed Maturity Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Sep. 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	$ 283,309
Available for sale securities, due after one year through five years, amortized cost	5,564,840
Available for sale securities, due after five years through ten years, amortized cost	9,893,201
Available for sale securities, due after ten years through twenty years, amortized cost	9,562,224
Available for sale securities, due after twenty years, amortized cost	8,598,718
Available for sale securities, securities with a single maturity date, amortized cost	33,902,292
Available for sale securities, amortized cost	45,688,342	$ 43,116,759
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available for sale securities, due in one year or less, fair value	287,677
Available for sale securities, due after one year through five years, fair value	5,561,473
Available for sale securities, due after five years through ten years, fair value	9,753,983
Available for sale securities, due after ten years through twenty years, fair value	9,883,734
Available for sale securities, due after twenty years, fair value	8,597,042
Available for sale securities, securities with a single maturity date, fair value	34,083,909
Available for sale securities, fair value	45,822,017	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	77,213
Held for investment securities, securities with a single maturity date, amortized cost	77,213
Held for investment, amortized cost	77,213	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	71,113
Held for investment securities, securities with a single maturity date, fair value	71,113
Held for investment	71,113	$ 76,460
Residential Mortgage Backed Securities
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Available for sale securities, securities without a single maturity date, amortized cost	1,129,176
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available for sale securities, securities without a single maturity date, fair value	1,181,682
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
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Available for sale securities, securities without a single maturity date, amortized cost	5,434,044
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available for sale securities, securities without a single maturity date, fair value	5,337,368
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
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Available for sale securities, securities without a single maturity date, amortized cost	5,222,830
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available for sale securities, securities without a single maturity date, fair value	5,219,058
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