Marketable securities - Schedule of Marketable Securities (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Current assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Market value	$ 0.9	$ 0.9
Cost basis	0.9	0.9
Unrealized gains, net
Noncurrent assets [Member]
Schedule of Available-for-sale Securities [Line Items]
Market value	256.7	256.8
Cost basis	256.6	256.7
Unrealized gains, net	0.1	0.1
Noncurrent assets [Member] \| Other [Member]
Schedule of Available-for-sale Securities [Line Items]
Market value	6.7	6.8
Cost basis	6.6	6.7
Unrealized gains, net	0.1	0.1
Noncurrent assets [Member] \| Amalgamated Sugar Company LLC [Member]
Schedule of Available-for-sale Securities [Line Items]
Market value	250.0	250.0
Cost basis	250.0	250.0
Unrealized gains, net

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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+ Details

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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