Summary of Investments - SOI Fixed Maturity AFS aging (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Amortized cost
Maturing in one year or less	$ 813,963
Maturing after one year through five years	3,808,805
Maturing after five years through ten years	8,164,513
Maturing after ten years	4,993,140
Mortgage-backed and asset-backed securities	5,040,850
Amortized cost	22,821,271	$ 22,762,962
Estimated fair value
Maturing in one year or less	826,233
Maturing after one year through five years	3,869,012
Maturing after five years through ten years	8,147,763
Maturing after ten years	5,270,972
Mortgage-backed and asset-backed securities	5,069,777
Estimated fair value	$ 23,183,757	$ 23,593,139