Segment Information - Schedule of consolidated revenue by geographic area (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue	$ 2,909	$ 2,340	$ 2,368	$ 2,381	$ 2,650	$ 2,201	$ 2,282	$ 2,276	$ 9,998	$ 9,409	$ 9,480
U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue									4,425	3,981	3,924
Americas other than U.S.
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue									976	899	1,118
U.K.
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue									1,436	1,354	1,419
Europe, Middle East, & Africa
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue									2,025	1,760	1,826
Asia Pacific
Revenues from External Customers and Long-Lived Assets [Line Items]
Total revenue									$ 1,136	$ 1,415	$ 1,193