Fair Value Measurement (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Dec. 31, 2010
Fair Value Measurement [Line Items]
Percentage of unadjusted fair value obtained	99.00%
Percentage of specific unadjusted broker quotes obtained	1.00%
Assets at fair value	$ 3,037,561		$ 3,159,497
Transfers between Levels 1, 2, and 3 of the fair value hierarchy	0	0
Nonrecurring measurements of nonfinancial assets or nonfinancial liabilities	0
Minimum [Member]
Fair Value Measurement [Line Items]
Number of knowledgeable outside security brokers consulted to determine fair value	1
Commercial Mortgage-Backed Securities [Member]
Fair Value Measurement [Line Items]
Assets at fair value	$ 0		$ 0