Fair Value Measurements (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Long-term Investments	$ 26,388,000		$ 26,388,000		$ 27,294,000
Realized investment gains (losses), net	4,859,000	$ 1,448,000	$ 5,151,000	$ 142,000
Limited Partnership Funds - Middle Market
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Maturity Timeframe of Other Long-Term Investments			10 years		10 years
Limited Partnership Funds - Term Liquidity Facility
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Maturity Timeframe of Other Long-Term Investments			0 years		3 years
Limited Partnership Funds - Late-Stage Growth
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Maturity Timeframe of Other Long-Term Investments			7 years		7 years
Limited Partnership Funds - Infrastructure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Estimated Maturity Timeframe of Other Long-Term Investments			12 years		12 years
Limited Partnership Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Long-term Investments	0		$ 0		$ 6,200,000
Realized investment gains (losses), net	4,000,000.0	$ (100,000)	4,500,000	$ (100,000)
Fair Value Measured at Net Asset Value Per Share | Limited Partnership Funds - Middle Market
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Long-term Investments	11,119,000		11,119,000		10,542,000
Fair Value Measured at Net Asset Value Per Share | Limited Partnership Funds - Middle Market | Unfunded Loan Commitment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Long-term Investments	29,205,000		29,205,000		29,783,000
Fair Value Measured at Net Asset Value Per Share | Limited Partnership Funds - Term Liquidity Facility
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Long-term Investments	0		0		1,381,000
Fair Value Measured at Net Asset Value Per Share | Limited Partnership Funds - Term Liquidity Facility | Unfunded Loan Commitment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Long-term Investments	0		0		0
Fair Value Measured at Net Asset Value Per Share | Limited Partnership Funds - Late-Stage Growth
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Long-term Investments	12,956,000		12,956,000		0
Fair Value Measured at Net Asset Value Per Share | Limited Partnership Funds - Late-Stage Growth | Unfunded Loan Commitment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Long-term Investments	11,202,000		11,202,000		16,291,000
Fair Value Measured at Net Asset Value Per Share | Limited Partnership Funds - Infrastructure
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Long-term Investments	1,948,000		1,948,000		0
Fair Value Measured at Net Asset Value Per Share | Limited Partnership Funds - Infrastructure | Unfunded Loan Commitment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Long-term Investments	18,652,000		18,652,000		17,497,000
Fair Value Measured at Net Asset Value Per Share | Limited Partnership Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Long-term Investments	26,023,000		26,023,000		11,923,000
Fair Value Measured at Net Asset Value Per Share | Limited Partnership Funds [Member] | Unfunded Loan Commitment
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other Long-term Investments	$ 59,059,000		$ 59,059,000		$ 63,571,000