Subsequent Events (Details) - USD ($) shares in Millions		1 Months Ended	12 Months Ended
	Feb. 06, 2018	Nov. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Feb. 02, 2018	Jan. 31, 2017	Jun. 30, 2014
Subsequent Event [Line Items]
Accelerated stock repurchase, amount authorized		$ 1,700,000,000.0	$ 8,500,000,000				$ 500,000,000	$ 1,500,000,000
Amount expended on share repurchase		$ 750,000,000	(750,000,000)
Available to repurchase common stock			1,430,000,000
Cash paid to acquire privately-held company, net of cash acquired			$ 60,449,000	$ 13,242,000	$ 95,331,000
Accelerated Stock Repurchase Program | Subsequent Event
Subsequent Event [Line Items]
Accelerated stock repurchase, amount authorized						$ 750,000,000
Accelerated stock repurchase, number of shares authorized (shares)						6.5
Available to repurchase common stock						$ 500,000,000
Cedexis, Inc. | Subsequent Event
Subsequent Event [Line Items]
Cash paid to acquire privately-held company, net of cash acquired	$ 66,500,000
Cash acquired from the acquisition	$ 6,200,000