Fair Value Measurement (Narrative) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) broker	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of portfolio of unadjusted fair values obtained			98.40%
Two private equity funds using NAV	$ 71,400,000		$ 71,400,000
One private equity fund using NAV	1,200,000		1,200,000
Fair Value, Assets, Level 1 to Level 2 to Level 3 Transfers, Amount	0	$ 0	$ 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			10 years
Borrowings [Member] | 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		$ 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	12,000,000.0		12,000,000.0		18,900,000
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure	4,554,532,000		4,554,532,000		4,317,826,000
Liabilities, fair value	1,356,000		1,356,000		77,000
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	3,513,548,000		3,513,548,000		3,156,091,000
Liabilities, fair value	0		0		0
Fair Value, Measurements, Recurring [Member] | Borrowings [Member] | Unsecured debt [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, fair value	406,144,000		406,144,000		394,279,000
Fair Value, Measurements, Recurring [Member] | Borrowings [Member] | Unsecured debt [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liabilities, fair value	$ 406,144,000		$ 406,144,000		394,279,000
Fair Value, Measurements, Recurring [Member] | Private Equity Funds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure					$ 1,200,000