Fair Value Level 3 Fixed Maturity Securities on a recurring basis by valuation source (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	$ 3,778,950	$ 3,279,885
Percent of Total	69.50%	70.50%
Commercial mortgage-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	$ 1,045,473	$ 878,566
Percent of Total	6.30%	15.00%
Other asset-backed
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	$ 714,521	$ 707,570
Percent of Total	11.70%	4.10%
Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	$ 29,123	$ 93,552
Fair Value, Measurements, Recurring | Corporate | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	6,588	22,011
Fair Value, Measurements, Recurring | Non-redeemable preferred stocks | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	6,927	6,862
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	7,702,628	7,033,045
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	$ 36,050	$ 100,414
Percent of Total	100.00%	100.00%
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring | Corporate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	$ 3,778,950	$ 3,279,885
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring | Corporate | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	6,588	22,011
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring | Commercial mortgage-backed | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	12,780	67,940
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring | Other asset-backed | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	9,755	3,601
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring | Non-redeemable preferred stocks
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	74,800	84,295
Estimate of Fair Value Measurement | Fair Value, Measurements, Recurring | Non-redeemable preferred stocks | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	6,927	6,862
Estimate of Fair Value Measurement | Third-party vendors | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	$ 20,780	$ 69,880
Percent of Total	57.60%	69.60%
Estimate of Fair Value Measurement | Third-party vendors | Fair Value, Measurements, Recurring | Corporate | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	$ 0	$ 1,940
Estimate of Fair Value Measurement | Third-party vendors | Fair Value, Measurements, Recurring | Commercial mortgage-backed | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	12,780	67,940
Estimate of Fair Value Measurement | Third-party vendors | Fair Value, Measurements, Recurring | Other asset-backed | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	8,000	0
Estimate of Fair Value Measurement | Third-party vendors | Fair Value, Measurements, Recurring | Non-redeemable preferred stocks | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	0	0
Estimate of Fair Value Measurement | Priced internally | Fair Value, Measurements, Recurring | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	$ 15,270	$ 30,534
Percent of Total	42.40%	30.40%
Estimate of Fair Value Measurement | Priced internally | Fair Value, Measurements, Recurring | Corporate | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	$ 6,588	$ 20,071
Estimate of Fair Value Measurement | Priced internally | Fair Value, Measurements, Recurring | Commercial mortgage-backed | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	0	0
Estimate of Fair Value Measurement | Priced internally | Fair Value, Measurements, Recurring | Other asset-backed | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	1,755	3,601
Estimate of Fair Value Measurement | Priced internally | Fair Value, Measurements, Recurring | Non-redeemable preferred stocks | Significant unobservable inputs (Level 3)
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Available-for-sale Securities	$ 6,927	$ 6,862