Summary of Investments - Fixed Maturity AFS aging (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Amortized cost
Maturing in one year or less	$ 624,398
Maturing after one year through five years	3,470,116
Maturing after five years through ten years	8,251,124
Maturing after ten years	5,419,415
Mortgage-backed and asset-backed securities	4,931,420
Amortized cost	22,696,473	$ 22,762,962
Estimated fair value
Maturing in one year or less	623,752
Maturing after one year through five years	3,451,165
Maturing after five years through ten years	8,071,691
Maturing after ten years	5,501,579
Mortgage-backed and asset-backed securities	4,902,115
Estimated fair value	$ 22,550,302	$ 23,593,139