Summary of Investments - Fixed Maturity AFS aging (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Amortized cost
Maturing in one year or less	$ 619,874
Maturing after one year through five years	3,447,149
Maturing after five years through ten years	8,216,510
Maturing after ten years	5,500,704
Mortgage-backed and asset-backed securities	4,973,335
Amortized cost	22,757,572	$ 22,762,962
Estimated fair value
Maturing in one year or less	620,381
Maturing after one year through five years	3,441,249
Maturing after five years through ten years	8,066,134
Maturing after ten years	5,670,372
Mortgage-backed and asset-backed securities	4,964,601
Estimated fair value	$ 22,762,737	$ 23,593,139