Investments - Net Investment Income (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Investment Income [Line Items]
Total net investment income	$ 357,596	$ 365,064	$ 376,507
Investment Portfolio
Net Investment Income [Line Items]
Investment expenses	(9,542)	(9,484)	(10,200)
Total net investment income	260,312	294,239	376,507
Deposit Asset On Reinsurance
Net Investment Income [Line Items]
Total net investment income	97,284	70,825	0
Fixed maturity securities \| Investment Portfolio
Net Investment Income [Line Items]
Investment income	232,917	283,228	353,303
Equity securities \| Investment Portfolio
Net Investment Income [Line Items]
Investment income	4,665	4,923	6,017
Limited partnership interests \| Investment Portfolio
Net Investment Income [Line Items]
Investment income	20,863	25,694	15,406
Short-term and other investments \| Investment Portfolio
Net Investment Income [Line Items]
Investment income	$ 11,409	$ (10,122)	$ 11,981

Represents the total of interest and dividend income, including any amortization and accretion (as applicable) of discounts and premiums, earned from (1) loans and leases whether held-for-sale or held-in-portfolio; (2) investment securities; (3) federal funds sold; (4) securities purchased under agreements to resell; (5) investments in banker's acceptances, commercial paper, or certificates of deposit; (6) dividend income; or (7) other investments not otherwise specified herein.

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