Long-Term Incentive Compensation - Compensation Plans (Details)	1 Months Ended	12 Months Ended							24 Months Ended
	Apr. 30, 2019 £ / shares	Dec. 31, 2019 GBP (£) £ / shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($)	Dec. 31, 2018 £ / shares	Dec. 31, 2017 USD ($)	Dec. 31, 2017 £ / shares	Dec. 31, 2016 $ / shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2019 £ / shares	Dec. 31, 2016 £ / shares
Share Plan
Performance hurdle related to Group TSR (as a percent)		95.00%	95.00%
Performance hurdle related to risk and sustainability metrics (as a percent)		5.00%	5.00%
Stock-based compensation plan expense			$ 73,700,000	$ 81,600,000		$ 66,800,000
DEP Funds - Liability settled			57,500,000	54,900,000		41,400,000
MFSA - liability settled			46,200,000	24,300,000		20,700,000
Profits interests and other			(3,900,000)	18,400,000		12,300,000
Social Security costs			10,800,000	9,400,000		10,300,000
Total charge to the Consolidated Statements of Comprehensive Income			184,300,000	188,600,000		151,500,000
Stock-based payments expense unrecognized			$ 59,600,000						$ 59,600,000
Stock-based payments expense, weighted average years		1 year 7 months 6 days	1 year 7 months 6 days
DEP Funds - Liability settled, Unrecognised compensation			$ 28,800,000
MFSA - liability settled, Unrecognized compensation			30,100,000
Profits interests, Unrecognized compensation			14,400,000
Social security costs, Unrecognised compensation			19,100,000
Total remaining charge to the Consolidated Statements of Comprehensive Income			$ 152,000,000.0
DEP Funds - Liability settled		1 year 3 months 18 days	1 year 3 months 18 days
MFSA - Liability settled		2 years 1 month 6 days	2 years 1 month 6 days
Profits interests and other		4 years 6 months	4 years 6 months
Social Security costs, weighted average years		10 months 24 days	10 months 24 days
Weighted average years, remaining charge to comprehensive income		1 year 9 months 18 days	1 year 9 months 18 days
Tranche 1
Share Plan
Additional holding period		2 years	2 years
Tranche 2
Share Plan
Additional holding period		1 year	1 year
Deferred Equity Plan
Share Plan
Stock-based compensation plan expense			$ 19,100,000	18,700,000		17,600,000
Stock-based payments expense unrecognized			$ 13,500,000						13,500,000
Stock-based payments expense, weighted average years		1 year 4 months 24 days	1 year 4 months 24 days
Restricted Share Plan
Share Plan
Vesting period		3 years	3 years
Stock-based compensation plan expense			$ 8,300,000	10,100,000		3,400,000
Stock-based payments expense unrecognized			$ 6,800,000						6,800,000
Stock-based payments expense, weighted average years		1 year 7 months 6 days	1 year 7 months 6 days
Buy As You Earn Share Plan
Share Plan
Maximum deduction from the gross salary (monthly) | £		£ 150
Stock-based compensation plan expense			$ 2,100,000	3,000,000.0		3,200,000
Stock-based payments expense unrecognized			$ 400,000						400,000
Stock-based payments expense, weighted average years		6 months	6 months
Sharesave Plan
Share Plan
Maximum deduction from the gross salary (monthly)			$ 312.50	312.50		312.50
Option exercise price | $ / shares								$ 31.20
Discount to the average share price		15.00%	15.00%
Market price at grant date | £ / shares		£ 2.15			£ 4.99		£ 75.28
Stock-based compensation plan expense			$ 100,000	$ 900,000		$ 800,000
Stock-based payments expense unrecognized			$ 700,000						700,000
Stock-based payments expense, weighted average years		2 years	2 years
Black Scholes options pricing model
Fair value of options granted (pence) | £ / shares		£ 2.15			4.99		75.28
Dividend yield (as a percent)		6.92%	6.92%	3.85%		3.99%
Expected volatility (as a percent)		30.17%	30.17%	32.20%		32.13%
Risk-free interest rate (as a percent)		0.55%	0.55%	0.70%		0.19%
Expected life (in years)		3 years	3 years	3 years		3 years
RSA
Share Plan
Stock-based compensation plan expense			$ 42,200,000	$ 44,900,000		$ 32,800,000
Stock-based payments expense unrecognized			$ 37,900,000						37,900,000
Stock-based payments expense, weighted average years		1 year 8 months 12 days	1 year 8 months 12 days
Company Share Option Plan
Share Plan
Option exercise price | £ / shares							22.80				£ 26.10
Exercisable period after vesting		3 months	3 months
Awards granted (in dollars)			$ 0
Market price at grant date | £ / shares							33.43
Stock-based compensation plan expense			300,000	600,000		$ 1,100,000
Stock-based payments expense unrecognized			$ 100,000						100,000
Stock-based payments expense, weighted average years		2 months 12 days	2 months 12 days
Black Scholes options pricing model
Fair value of options granted (pence) | £ / shares							33.43
Dividend yield (as a percent)						4.64%
Expected volatility (as a percent)						32.41%
Risk-free interest rate (as a percent)						0.27%
Expected life (in years)						3 years
US CSOP
Share Plan
Market price at grant date | £ / shares							32.81
Black Scholes options pricing model
Fair value of options granted (pence) | £ / shares							32.81
Dividend yield (as a percent)						4.64%
Expected volatility (as a percent)						35.19%
Risk-free interest rate (as a percent)						0.16%
Expected life (in years)						2 years
Executive Shared Ownership Plan
Share Plan
Market price at grant date | £ / shares							27.78
Stock-based compensation plan expense			$ 300,000	800,000		$ 1,500,000
Stock-based payments expense unrecognized			$ 100,000						100,000
Stock-based payments expense, weighted average years		2 months 12 days	2 months 12 days
Black Scholes options pricing model
Fair value of options granted (pence) | £ / shares							27.78
Dividend yield (as a percent)						4.64%
Expected volatility (as a percent)						32.41%
Risk-free interest rate (as a percent)						0.27%
Expected life (in years)						3 years
2017 ExSOP
Share Plan
Vesting period		3 years	3 years
Exercisable period after vesting		2 years	2 years
Awards granted (in dollars)									0
Market price at grant date | £ / shares							22.62
Hurdle price | £ / shares							24.90
Black Scholes options pricing model
Fair value of options granted (pence) | £ / shares							22.62
2016 ExSOP
Share Plan
Market price at grant date | £ / shares	£ 26.14
Hurdle price | £ / shares	28.45
Black Scholes options pricing model
Fair value of options granted (pence) | £ / shares	£ 26.14
LongTerm Incentive Plan
Share Plan
Option exercise price | £ / shares										£ 0
Stock-based compensation plan expense			$ 1,300,000	$ 2,600,000		$ 6,400,000
Stock-based payments expense unrecognized			$ 100,000						$ 100,000
Stock-based payments expense, weighted average years		2 months 12 days	2 months 12 days
2013 LTIP
Share Plan
Vesting period		3 years	3 years
Options exercise period		5 years	5 years
2014 LTIP | Minimum
Share Plan
Vesting period		3 years	3 years
2014 LTIP | Maximum
Share Plan
Vesting period		4 years	4 years
2014 LTIP | Tranche 1
Share Plan
Awards to be vested (in percentage)								3.00%
2014 LTIP | Tranche 2
Share Plan
Awards to be vested (in percentage)				3.00%
2015 LTIP
Share Plan
Vesting period		3 years	3 years
Award exercisable percentage from end of year four		33.00%	33.00%
2015 LTIP | Tranche 1
Share Plan
Awards to be vested pre-Merger (in percentage)						25.00%
Awards to be vested post-Merger (in percentage)						74.60%
Monte Carlo Model
Share price at DoM		233.70%	233.70%
Risk free discount rate		0.10%	0.10%
Dividend yield		4.50%	4.50%
Share price volatility		30.00%	30.00%
Holding period adjustment		9.0	9.0
Percentage based on pre-modification performance conditions		80.00%	80.00%
2015 LTIP | Tranche 2
Share Plan
Awards to be vested pre-Merger (in percentage)				25.00%
Awards to be vested post-Merger (in percentage)				35.50%
Monte Carlo Model
Share price at DoM		233.70%	233.70%
Risk free discount rate		0.10%	0.10%
Dividend yield		4.50%	4.50%
Share price volatility		30.00%	30.00%
Holding period adjustment		6.2	6.2
Percentage based on pre-modification performance conditions		60.00%	60.00%
2016 LTIP
Share Plan
Vesting period		4 years	4 years
Award exercisable percentage from end of year three		67.00%	67.00%
Award exercisable percentage from end of year four		33.00%	33.00%
2016 LTIP | Tranche 1
Share Plan
Awards to be vested pre-Merger (in percentage)				25.00%
Awards to be vested post-Merger (in percentage)				35.50%
Monte Carlo Model
Share price at DoM		233.70%	233.70%
Risk free discount rate		0.10%	0.10%
Dividend yield		4.50%	4.50%
Share price volatility		30.00%	30.00%
Holding period adjustment		9.0	9.0
2016 LTIP | Tranche 2
Monte Carlo Model
Share price at DoM		233.70%	233.70%
Risk free discount rate		0.10%	0.10%
Dividend yield		4.50%	4.50%
Share price volatility		30.00%	30.00%
Holding period adjustment		6.2	6.2
Relative TSR
Share Plan
Weight of awards criteria in performance conditions		50.00%	50.00%
Relative TSR | 2015 LTIP
Monte Carlo Model
Allocation of award (as a percent)		50.00%	50.00%
Relative TSR | 2015 LTIP | Tranche 1
Monte Carlo Model
Allocation of award (as a percent)		118.96%	118.96%
Relative TSR | 2015 LTIP | Tranche 2
Monte Carlo Model
Allocation of award (as a percent)		124.11%	124.11%
Relative TSR | 2016 LTIP
Monte Carlo Model
Allocation of award (as a percent)		50.00%	50.00%
Relative TSR | 2016 LTIP | Tranche 1
Monte Carlo Model
Allocation of award (as a percent)		120.98%	120.98%
Relative TSR | 2016 LTIP | Tranche 2
Monte Carlo Model
Allocation of award (as a percent)		123.64%	123.64%
Relative investment performance
Share Plan
Weight of awards criteria in performance conditions		25.00%	25.00%
Relative investment performance | 2015 LTIP
Monte Carlo Model
Allocation of award (as a percent)		25.00%	25.00%
Relative investment performance | 2015 LTIP | Tranche 1
Monte Carlo Model
Allocation of award (as a percent)		209.76%	209.76%
Relative investment performance | 2015 LTIP | Tranche 2
Monte Carlo Model
Allocation of award (as a percent)		206.59%	206.59%
Relative investment performance | 2016 LTIP
Monte Carlo Model
Allocation of award (as a percent)		25.00%	25.00%
Relative investment performance | 2016 LTIP | Tranche 1
Monte Carlo Model
Allocation of award (as a percent)		200.42%	200.42%
Relative investment performance | 2016 LTIP | Tranche 2
Monte Carlo Model
Allocation of award (as a percent)		197.39%	197.39%
Relative net income before tax growth
Share Plan
Weight of awards criteria in performance conditions		25.00%	25.00%
Relative net income before tax growth | 2015 LTIP
Monte Carlo Model
Allocation of award (as a percent)		25.00%	25.00%
Relative net income before tax growth | 2015 LTIP | Tranche 1
Monte Carlo Model
Allocation of award (as a percent)		209.76%	209.76%
Relative net income before tax growth | 2015 LTIP | Tranche 2
Monte Carlo Model
Allocation of award (as a percent)		206.59%	206.59%
Relative net income before tax growth | 2016 LTIP
Monte Carlo Model
Allocation of award (as a percent)		25.00%	25.00%
Relative net income before tax growth | 2016 LTIP | Tranche 1
Monte Carlo Model
Allocation of award (as a percent)		200.42%	200.42%
Relative net income before tax growth | 2016 LTIP | Tranche 2
Monte Carlo Model
Allocation of award (as a percent)		197.39%	197.39%
FTSE 350
Share Plan
Weight of awards criteria in performance conditions		25.00%	25.00%
ASX 100
Share Plan
Weight of awards criteria in performance conditions		25.00%	25.00%
Net fund flows condition
Share Plan
Weight of awards criteria in performance conditions		15.00%	15.00%
Investment performance condition
Share Plan
Weight of awards criteria in performance conditions		15.00%	15.00%
Operating margin condition
Share Plan
Weight of awards criteria in performance conditions		15.00%	15.00%
People strategy condition
Share Plan
Weight of awards criteria in performance conditions		10.00%	10.00%
UK employees | Sharesave Plan
Share Plan
Vesting period		3 years	3 years
Maximum deduction from the gross salary (monthly) | £		£ 500
Option exercise price | £ / shares					£ 20.16		£ 18.40			£ 14.76
Discount to the average share price		20.00%	20.00%
Average share price in prior period		5 days	5 days
Exercisable period after vesting		6 months	6 months
UK employees | Company Share Option Plan
Share Plan
Vesting period		3 years	3 years
Exercisable period after vesting		2 years	2 years
Maximum value of unvested options approved | shares			30,000						30,000