Summary of Goodwill and Acquired Intangible Assets (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2012
Finite-Lived Intangible Assets, Net [Abstract]
Goodwill, gross amount, beginning balance	$ 269,375
Goodwill, additions	56,251	[1]
Goodwill, impairment loss	0
Goodwill, gross amount, ending balance	325,626
Accumulated impairment losses, beginning balance	(8,879)
Accumulated impairment losses, ending balance	(8,879)
Goodwill, net, beginning balance	260,496
Goodwill, net, additions	56,251
Goodwill, net, ending balance	$ 316,747

[1] Goodwill associated with the acquisition of substantially all of the operating assets and contract rights of Health Plus on May 1, 2012 (see Note 6).