Investments (Schedule of Contractual Maturities of Fixed Maturity Investments) (Details) $ in Thousands	Mar. 31, 2018 USD ($)
Trading Securities [Abstract]
Amortized Cost, Due in less than one year	$ 496,788
Amortized Cost, Due after one through five years	4,381,056
Amortized Cost, Due after five through ten years	1,073,453
Amortized Cost, Due after ten years	145,438
Amortized Cost	7,470,003
Fair Value, Due in less than one year	495,143
Fair Value, Due after one through five years	4,324,744
Fair Value, Due after five through ten years	1,064,575
Fair Value, Due after ten years	143,696
Fair Value	7,404,761
Mortgage-backed
Trading Securities [Abstract]
Amortized Cost	1,189,730
Fair Value	1,193,141
Asset-backed
Trading Securities [Abstract]
Amortized Cost	183,538
Fair Value	$ 183,462