Fair Value Measurement (Narrative) (Details)	3 Months Ended
	Mar. 31, 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	98.00%
Assets, fair value	$ 3,729,640,000	$ 3,738,293,000
Assets, Fair Value Disclosure, Nonrecurring	0
Liabilities, Fair Value Disclosure, Nonrecurring	0
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	2,970,251,000	2,921,995,000
Commercial Mortgage Backed Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading securities, fair value disclosure	$ 19,100,000	19,300,000
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
Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	$ 549,786,000	537,240,000
Equity Securities [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	34,320,000	34,869,000
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
Private Equity Funds, Net Asset Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	1,500,000	1,500,000
Private Equity Funds, Net Asset Value [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Assets, fair value	74,615,000	$ 71,523,000
Unfunded commitments	$ 0
Private Equity Funds, Net Asset Value [Member] | Equity Securities [Member] | Minimum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liquidating investment, remaining period	1 year
Private Equity Funds, Net Asset Value [Member] | Equity Securities [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Liquidating investment, remaining period	5 years