Employee benefit plans - Composition of Pension Plan Assets (Detail) (Defined Benefit Pension Plans [Member], USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	$ 460.6	$ 389.1
Other Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	19.5	16.9
Germany [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	224.8	187.2
Canada [Member] | Local currency equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	22.4	18.1
Canada [Member] | Foreign equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	30.3	28.0
Canada [Member] | Local currency fixed income [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	38.0	33.8
Canada [Member] | Cash and other [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	2.1	2.4
Canada [Member] | Global Mutual Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	5.6
Norway [Member] | Local currency equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	3.2	2.0
Norway [Member] | Foreign equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	5.2	3.7
Norway [Member] | Local currency fixed income [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	40.9	35.9
Norway [Member] | Cash and other [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	7.6	6.3
Norway [Member] | Foreign fixed income [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	4.8	4.3
Norway [Member] | Real estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	5.5	5.1
United States [Member] | CMRT [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	50.7	45.4
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	171.8	143.2
Level 1 [Member] | Other Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	12.3	10.0
Level 1 [Member] | Canada [Member] | Local currency equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	22.4	18.1
Level 1 [Member] | Canada [Member] | Foreign equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	30.3	28.0
Level 1 [Member] | Canada [Member] | Local currency fixed income [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	38.0	33.8
Level 1 [Member] | Canada [Member] | Cash and other [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	2.1	2.4
Level 1 [Member] | Canada [Member] | Global Mutual Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	5.6
Level 1 [Member] | Norway [Member] | Local currency equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	3.2	2.0
Level 1 [Member] | Norway [Member] | Foreign equities [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	5.2	3.7
Level 1 [Member] | Norway [Member] | Local currency fixed income [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	40.9	35.9
Level 1 [Member] | Norway [Member] | Cash and other [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	7.0	4.9
Level 1 [Member] | Norway [Member] | Foreign fixed income [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	4.8	4.3
Level 1 [Member] | United States [Member] | CMRT [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets		0.1
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	50.7	45.3
Level 2 [Member] | United States [Member] | CMRT [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	50.7	45.3
Level 3 Inputs [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	238.1	200.6	214.9
Level 3 Inputs [Member] | Other Pension Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	7.2	6.9
Level 3 Inputs [Member] | Germany [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	224.8	187.2
Level 3 Inputs [Member] | Norway [Member] | Cash and other [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	0.6	1.4
Level 3 Inputs [Member] | Norway [Member] | Real estate [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets	$ 5.5	$ 5.1