Investments - Clean Coal Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014 Plant	Dec. 31, 2013	Dec. 31, 2012	Sep. 01, 2013	Mar. 01, 2014	Mar. 01, 2013 Plant
Schedule of Equity Method Investments [Line Items]
Total revenues	$ 1,245.4	$ 1,286.8	$ 1,179.3	$ 915.0	$ 890.2	$ 835.8	$ 779.5	$ 674.1	$ 4,626.5	$ 3,179.6	$ 2,520.3
Total expenses	(1,219.1)	(1,199.1)	(1,072.2)	(868.7)	(840.9)	(750.3)	(682.1)	(631.8)	4,359.1	2,905.1	2,275.0
Ownership interest acquired in a limited liability company	50.00%								50.00%			99.00%
Amount of promissory note												10.0
Purchase price paid in cash												4.0
Number of clean coal production plants seeking and negotiating for long term production contract									8
Limited Liability Company [Member]
Schedule of Equity Method Investments [Line Items]
Carrying value of prior non-controlling interest in limited liability company on acquisition date													15.6
Ownership interest acquired in a limited liability company	50.00%								50.00%
Notes Payable, Other Payables [Member]
Schedule of Equity Method Investments [Line Items]
Interest rate on promissory note												3.00%
2009 Era Plants [Member]
Schedule of Equity Method Investments [Line Items]
Number of additional plants acquired													7	12
Number of additional plants acquired with ownership interest range one														9
Number of additional plants acquired with ownership interest range two														3
Ownership interest before additional nine plants acquired														24.50%
Ownership interest after additional nine plants acquired														49.50%
Ownership interest before additional three plants acquired														25.00%
Ownership interest after additional three plants acquired														60.00%
Total revenues									260.9	128.3
Total expenses									264.3	133.5
Purchase of consideration received	8.0								8.0
Payment of cash and other consideration	5.0								5.0
Gain on purchase of additional interest	11.4								11.4
Prior percentage of equity interest in limited liability company	25.00%								25.00%
Additional percentage of equity interest in limited liability company upon acquisition	35.00%								35.00%
Fixed assets and other amortizable intangible assets acquired upon acquisition	26.3								26.3
Carrying value of prior non-controlling interest in limited liability company on acquisition date	4.8								4.8
Other assets acquired upon acquisition	6.8								6.8
Ownership interest before additional seven plants acquired													49.50%
Ownership interest after additional seven plants acquired													100.00%
2011 Era Plants [Member]
Schedule of Equity Method Investments [Line Items]
Number of additional plants acquired													5
Total revenues									381.6
Total expenses									405.7
Gain on purchase of additional interest													25.6
Fixed assets and other amortizable intangible assets acquired upon acquisition													26.3
Ownership interest before additional one plant acquired													48.80%
Ownership interest after additional one plant acquired													90.00%
Ownership interest before additional three plants acquired													49.00%
Ownership interest after additional three plants acquired													100.00%
Ownership interest before additional one plant acquired													98.00%
Ownership interest after additional one plant acquired													100.00%
2011 Era Plants [Member] | Notes Payable, Other Payables [Member]
Schedule of Equity Method Investments [Line Items]
Total revenues									84.0	33.7
Total expenses									93.0	36.9
Chem-Mod LLC [Member]
Schedule of Equity Method Investments [Line Items]
Number of clean coal production plants owned									34
Chem-Mod LLC [Member] | 2009 Era Plants [Member]
Schedule of Equity Method Investments [Line Items]
Number of clean coal production plants owned									14
Chem-Mod LLC [Member] | 2011 Era Plants [Member]
Schedule of Equity Method Investments [Line Items]
Number of clean coal production plants owned									20
Long Term Production Contracts [Member]
Schedule of Equity Method Investments [Line Items]
Number of clean coal production plants owned									26
Clean-Coal Investments [Member]
Schedule of Equity Method Investments [Line Items]
Number of variable interest entities	1								1
Clean-Coal Investments [Member] | VIE [Member]
Schedule of Equity Method Investments [Line Items]
Total assets of limited liability companies	4.4								4.4
Total liability of limited liability companies	1.4								1.4
Total revenues of limited liability companies									33.4
Total expenses of limited liability companies									$ 41.0