Fair Value Measurement (Narrative) (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Percentage of unadjusted fair value obtained	98.00%
Percentage of specific unadjusted broker quotes obtained	2.00%
Assets at fair value	$ 3,118,504		$ 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]
Number of knowledgeable outside security brokers consulted to determine fair value	1
Commercial Mortgage-Backed Securities [Member]
Assets at fair value			$ 0