Fair Value Measurement (Narrative) (Details)	9 Months Ended
	Sep. 30, 2021 USD ($) broker	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of portfolio of unadjusted fair values obtained	97.90%
Two private equity funds using NAV	$ 105,000,000
One private equity fund using NAV	1,200,000
Fair Value, Assets, Level 1 to Level 2 to Level 3 Transfers, Amount	$ 0	$ 0
Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of knowledgeable outside security brokers consulted to determine fair value | broker	1
Fair Value, liquidating investment, remaining period	1 year
Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value, liquidating investment, remaining period	9 years
Unsecured debt [Member] | Level 2 [Member]
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 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Debt securities, trading, and equity securities, FV-NI	25,600,000		17,600,000
Private Equity Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unfunded commitments to VIEs	40,000,000		0
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure	5,080,495,000		4,734,995,000
Liabilities	473,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure	4,062,709,000		$ 3,579,308,000
Liabilities	$ 0