Revenue - Schedule of Revenue by Geography (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 1,826	$ 6,066
Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	1,816	6,013
Corporate, Non-Segment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	4	13
North America [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	946	2,857
North America [Member] | Corporate, Non-Segment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	4	13
Great Britain [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	389	1,433
Great Britain [Member] | Corporate, Non-Segment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0	0
Western Europe [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	264	1,024
Western Europe [Member] | Corporate, Non-Segment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0	0
International [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	217	699
International [Member] | Corporate, Non-Segment [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0	0
HCB [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	787	2,421
HCB [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	773	2,376
HCB [Member] | North America [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	466	1,395
HCB [Member] | Great Britain [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	113	362
HCB [Member] | Western Europe [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	121	399
HCB [Member] | International [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	73	220
CRB [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	606	2,007
CRB [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	606	2,007
CRB [Member] | North America [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	266	740
CRB [Member] | Great Britain [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	137	455
CRB [Member] | Western Europe [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	96	456
CRB [Member] | International [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	107	356
IRR [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	308	1,254
IRR [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	306	1,249
IRR [Member] | North America [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	83	341
IRR [Member] | Great Britain [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	139	616
IRR [Member] | Western Europe [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	47	169
IRR [Member] | International [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	37	123
BDA [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	128	373
BDA [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	127	368
BDA [Member] | North America [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	127	368
BDA [Member] | Great Britain [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0	0
BDA [Member] | Western Europe [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	0	0
BDA [Member] | International [Member] | Operating Segments [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax	$ 0	$ 0