Fair Value Measurement (Narrative) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($) broker	Dec. 31, 2015 USD ($)
Fair Value Measurement [Line Items]
Percentage of portfolio of unadjusted fair values obtained	99.70%
Percentage of portfolio of specific unadjusted broker quotes obtained	0.30%
Fair value of assets measured on a recurring basis	$ 3,548,227	$ 3,380,642
Level 3 [Member]
Fair Value Measurement [Line Items]
Fair value of assets measured on a recurring basis	9,068	10,431
Level 2 [Member]
Fair Value Measurement [Line Items]
Fair value of assets measured on a recurring basis	2,854,987	2,892,014
Level 2 [Member] | Commercial Mortgage Backed Securities [Member]
Fair Value Measurement [Line Items]
Trading securities, fair value disclosure	$ 30,000	$ 37,300
Minimum [Member]
Fair Value Measurement [Line Items]
Number of knowledgeable outside security brokers consulted to determine fair value | broker	1