Stock Benefit Plans (Tables)

12 Months Ended

Dec. 31, 2011

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Summary of the status of MMC's stock option awards

	Shares	Weighted Average Exercise Price		Weighted Average Remaining Contractual Term	Aggregate Intrinsic Value ($000)
Balance at January 1, 2011	43,386,136	$	29.33
Granted	2,827,655	$	30.60
Exercised	4,255,079	$	26.26
Canceled or exchanged	—	—
Forfeited	294,840	$	25.46
Expired	2,768,763	$	37.12
Balance at December 31, 2011	38,895,109	$	29.21	4.6 years	155,159
Options vested or expected to vest at December 31, 2011	38,030,022	$	29.38	4.8 years	221,216
Options exercisable at December 31, 2011	12,913,460	$	31.47	3.0 years	58,228

Restricted Stock Units And Performance Stock Units [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Summary of restricted stock units award activity

	Restricted Stock Units			Performance Units
	Shares	Weighted Average Grant Date Fair Value		Shares	Weighted Average Grant Date Fair Value
Non-vested balance at January 1, 2011	18,749,101	$	22.91	—	$	—
Granted	5,549,028	$	30.46	390,771	$	30.60
Vested	8,465,116	$	23.16	2,809	$	30.60
Forfeited	774,585	$	25.12	19,616	$	30.60
Non-vested balance at December 31, 2011	15,058,428	$	25.43	368,346	$	30.60

Deferred Stock Units [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Summary of restricted stock units award activity

	Shares	Weighted Average Grant Date Fair Value
Non-vested balance at January 1, 2011	73,900	$	41.41
Granted	—	$	—
Vested	22,200	$	28.74
Forfeited	—	$	—
Non-vested balance at December 31, 2011	51,700	$	46.86

Black-Scholes Option Pricing [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Option pricing valuation model for options granted

	2011	2010	2009
Risk-free interest rate	2.28%-2.90%	3.15%-3.20%	2.16%-2.68%
Expected life (in years)	6.75	6.75	6.75
Expected volatility	25.4%-25.8%	26.3%-27.6%	33.5%-36.4%
Expected dividend yield	2.75%-2.86%	3.26%-3.52%	4.15%-4.20%

Binomial Option Pricing [Member]

Share-based Compensation Arrangement by Share-based Payment Award [Line Items]

Option pricing valuation model for options granted

	2009
Risk-free interest rate	0.42%-2.40%
Expected life (in years)	5.6-7.7
Expected volatility	35.6%
Expected dividend yield	3.62%