Fair Value Measurement (Narrative) (Details)	9 Months Ended
	Sep. 30, 2019 USD ($) broker	Dec. 31, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of portfolio of unadjusted fair values obtained	98.50%
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
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
Commercial Mortgage Back Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities, fair value disclosure	21,000,000.0	24,800,000
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure	0
Liabilities, fair value	0
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure	4,223,801,000	3,773,648,000
Liabilities, fair value	1,404,000	4,854,000
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	3,193,721,000	3,013,932,000
Liabilities, fair value	1,167,000	4,851,000
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	394,129,000	362,674,000
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	394,129,000	362,674,000
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	$ 1,400,000