Investments (Schedule of Contractual Maturities of Fixed Maturity Investments) (Details) $ in Thousands	Mar. 31, 2017 USD ($)
Trading Securities [Abstract]
Amortized Cost, Due in less than one year	$ 516,733
Amortized Cost, Due after one through five years	4,014,681
Amortized Cost, Due after five through ten years	1,173,318
Amortized Cost, Due after ten years	193,136
Amortized Cost	7,261,140
Fair Value, Due in less than one year	514,037
Fair Value, Due after one through five years	4,004,237
Fair Value, Due after five through ten years	1,182,697
Fair Value, Due after ten years	192,570
Fair Value	7,259,851
Mortgage-backed
Trading Securities [Abstract]
Amortized Cost	1,157,303
Fair Value	1,159,686
Asset-backed
Trading Securities [Abstract]
Amortized Cost	205,969
Fair Value	$ 206,624