Deferred Policy Acquisition Costs, Deferred Sales Inducements and Liability for Lifetime Income Benefit Riders (Liability for Lifetime Income Benefit Riders) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Liability for Future Policy Benefit, by Product Segment [Line Items]
Balance at beginning of year	$ 57,606,009
Balance at end of year	$ 61,893,945	$ 57,606,009
Experience study on products with lifetime income benefit riders, period	10 years
Assumptions for future crediting rates, US Treasury rate	3.80%
Mean reversion period	20 years
Aggregate spread assumption, rate	2.60%
Ultimate discount rate assumption	2.90%
Liability for Lifetime Income Benefit Riders
Liability for Future Policy Benefit, by Product Segment [Line Items]
Balance at beginning of year	$ 808,167	704,441	$ 533,391
Benefit expense accrual	179,901	157,333	149,442
Impact of unlocking	315,383	(53,607)	21,608
Claim payments	0	0	0
Balance at end of year	$ 1,303,451	$ 808,167	$ 704,441
Lifetime Income Benefit Rider, Fee Rider
Liability for Future Policy Benefit, by Product Segment [Line Items]
Ultimate utilization assumption, rate	60.00%	75.00%
Lifetime Income Benefit Rider, No Fee Rider
Liability for Future Policy Benefit, by Product Segment [Line Items]
Ultimate utilization assumption, rate	30.00%	37.50%