Fair Value Measurement (Narrative) (Details) - USD ($)	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of portfolio of unadjusted fair values obtained	97.80%
Three private equity funds using NAV	$ 92,100,000
Fair Value, Assets, Level 1 to Level 2 to Level 3 Transfers, Amount	$ 0	$ 0
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, liquidating investment, remaining period	1 year
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, liquidating investment, remaining period	10 years
Unsecured debt | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unsecured notes	$ 375,000,000		$ 375,000,000
Commercial Mortgage Back Securities [Member] | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, trading, and equity securities, FV-NI	27,800,000		25,200,000
Private Equity Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unfunded commitments to VIEs	10,000,000		32,000,000
Venture Capital Investments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
One private equity fund using NAV	100,000
Technology, Healthcare, Specialty Customer Goods and Service
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
One private equity fund using NAV	600,000
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure	4,682,815,000		5,142,589,000
Liabilities	169,000		301,000
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure	3,892,574,000		4,099,687,000
Liabilities	$ 0		$ 0