Fair Value - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes	$ 1,871,626	$ 1,733,528
Total assets and liabilities measured at fair value	22,900,000	20,400,000
Total assets and liabilities measured at fair value priced using non-binding broker quotes	$ 179,600	$ 217,900
Total assets and liabilities measured at fair value priced using non-binding broker quotes (percentage)	0.80%	1.10%
Unsecured debt | Senior Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Senior notes	$ 1,870,000	$ 1,730,000
Estimated fair value of senior notes	2,340,000	$ 1,880,000
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset Transfers Into Level 3	107,400
Other investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset Transfers Into Level 3	31,600
Corporate bonds | Fixed maturities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Asset Transfers Into Level 3	$ 25,800