Employee benefit plans - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Contribution Plan [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Defined contribution plan expense	$ 3.9	$ 3.4	$ 2.9
Defined Benefit Pension Plans [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Expected contribution	28.5
Expect to recognize unrecognized actuarial losses	13.7
Expect to recognize prior service credit\cost	1.1
Expect to recognize transition obligations, Net	0.4
Plan assets	460.6	389.1
Prior service credit arising from plan amendment			(3.8)
Defined Benefit Pension Plans [Member] | Level 3 Inputs [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Plan assets	238.1	200.6	214.9
Defined Benefit Pension Plans [Member] | Norway [Member] | Real estate [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Plan assets	5.5	5.1
Defined Benefit Pension Plans [Member] | Norway [Member] | Real estate [Member] | Level 3 Inputs [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Plan assets	5.5	5.1
U.S. defined benefit pension plans [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate	3.60%	4.20%
Percentage of assumed long-term rate of return for plan assets invested	10.00%	10.00%	10.00%
Plan assets	50.7	45.4	50.4
U.S. defined benefit pension plans [Member] | CMRT [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Percentage of average annual rate of return	14.00%
Percentage of assumed long-term rate of return for plan assets invested	10.00%	10.00%	10.00%
U.S. defined benefit pension plans [Member] | Level 1 and Level 2 [Member] | CMRT [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Percentage of fair value of asset	83.00%	83.00%
U.S. defined benefit pension plans [Member] | Plan assets attributable to disposition of certain investments [Member] | CMRT [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Plan assets	254.7
Percentage of aggregate assets value	35.00%
Foreign Pension Plan Defined Benefit [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate	3.70%	4.90%
Percentage of assumed long-term rate of return for plan assets invested	5.20%	5.50%	5.50%
Plan assets	409.9	343.7	334.8
Foreign Pension Plan Defined Benefit [Member] | Canada [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Average basis points of long-term rate of return on investment	125
Foreign Pension Plan Defined Benefit [Member] | Canada [Member] | Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Percentage of plan asset allocation	46.00%
Foreign Pension Plan Defined Benefit [Member] | Canada [Member] | Fixed income securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Percentage of plan asset allocation	45.00%
Foreign Pension Plan Defined Benefit [Member] | Norway [Member] | Level 3 Inputs [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Percentage of plan asset allocation	10.00%
Foreign Pension Plan Defined Benefit [Member] | Norway [Member] | Equity securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Percentage of assumed long-term rate of return for plan assets invested	8.00%
Percentage of plan asset allocation	12.00%
Foreign Pension Plan Defined Benefit [Member] | Norway [Member] | Fixed income securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Percentage of assumed long-term rate of return for plan assets invested	4.00%
Percentage of plan asset allocation	78.00%
Foreign Pension Plan Defined Benefit [Member] | Norway [Member] | Real estate [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Percentage of assumed long-term rate of return for plan assets invested	6.00%
Percentage of plan asset allocation	9.00%
Foreign Pension Plan Defined Benefit [Member] | Norway [Member] | Money Markets [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Percentage of assumed long-term rate of return for plan assets invested	7.00%
OPEB [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Expected contribution	1.6
Discount rate	3.47%	3.93%
Expect to recognize prior service credit\cost	1.8
Prior service credit arising from plan amendment			$ 9.5
weighted average discount rate used in determining the net periodic cost	5.40%	4.65%	3.93%