Investments (Amortized Cost and Fair Value of Fixed Maturities by Contractual Maturities) (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Amortized Cost
Due in one year or less	$ 628,670
Due after one year through five years	1,277,425
Due after five years through ten years	1,313,341
Due after ten years	5,874,893
Amortized Cost	10,078,359	$ 9,818,298
Fair Value
Due in one year or less	635,530
Due after one year through five years	1,299,750
Due after five years through ten years	1,353,243
Due after ten years	5,408,069
Fair Value	9,687,098	$ 9,362,763
Asset-backed securities
Amortized Cost
Debt Maturities, without single maturity date	312,110
Fair Value
Debt Maturities, without Single Maturity Date	315,595
Commercial mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	484,174
Fair Value
Debt Maturities, without Single Maturity Date	484,133
Residential mortgage-backed securities
Amortized Cost
Debt Maturities, without single maturity date	187,746
Fair Value
Debt Maturities, without Single Maturity Date	$ 190,778