Share-based Compensation Plans (Performance Share Awards) (Details) - RSUs, PSUs and PSAs	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Number of shares
Outstanding as of beginning of period (in shares) | shares	3,976,657
Granted (in shares) | shares	2,520,239
Exercised (in shares) | shares	(1,779,205)
Forfeited (in shares) | shares	(757,971)
Outstanding as of end of period (in shares) | shares	3,959,720
Exercisable as of end of period (in shares) | shares	0
Weighted average grant date fair value
Outstanding as of beginning of period (in GBP per share) | $ / shares	$ 41.31
Granted (in GBP per share) | $ / shares	45.38
Forfeited (in GBP per share) | $ / shares	44.99
Exercised (in GBP per share) | $ / shares	43.23
Outstanding as of end of period (in GBP per share) | $ / shares	42.33
Exercisable as of end of period (in GBP per share) | $ / shares	$ 0.00
Weighted average remaining life	4 years 10 months 25 days