Fair Value Measurement (Narrative) (Details)	9 Months Ended
	Sep. 30, 2018 USD ($) broker	Dec. 31, 2017 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percentage of portfolio of unadjusted fair values obtained	98.20%
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	25,000,000	19,300,000
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value disclosure	3,909,578,000	3,738,293,000
Liabilities, fair value	1,550,000	1,323,000
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	3,001,954,000	2,921,995,000
Liabilities, fair value	961,000	1,200,000
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	361,118,000	385,583,000
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	361,118,000	385,583,000
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	1,400,000	$ 1,500,000
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