Income Taxes - Reconciliation of Provision for Income Taxes with Federal Statutory Income Tax Rate (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Federal statutory rate	$ 93.6	$ 96.1	$ 85.9
State income taxes - net of Federal benefit	12.0	6.5	9.4
Foreign taxes	0.8	(0.8)	0.9
Alternative energy, foreign and other tax credits	(145.5)	(93.8)	(45.3)
Foreign dividends and other permanent differences	(6.1)	(2.5)	(2.7)
Nondeductible employee compensation	5.4
Changes in unrecognized tax benefits	2.4	1.5	0.6
Change in valuation allowance		0.5	0.3
Other	1.4	(1.6)	1.2
Provision (benefit) for income taxes	$ (36.0)	$ 5.9	$ 50.3
Federal statutory rate, % of Pretax Earnings	35.00%	35.00%	35.00%
State income taxes-net of Federal benefit, % of Pretax Earnings	4.50%	2.40%	3.80%
Foreign taxes, % of Pretax Earnings	0.30%	(0.30%)	0.40%
Alternative energy, foreign and other tax credits, % of Pretax Earnings	(54.40%)	(34.20%)	(18.50%)
Foreign dividends and other permanent differences, % of Pretax Earnings	(2.30%)	(0.90%)	(1.10%)
Nondeductible employee compensation, % of Pretax Earnings	2.00%
Changes in unrecognized tax benefits, % of Pretax Earnings	0.90%	0.50%	0.20%
Change in valuation allowance, % of Pretax Earnings		0.20%	0.10%
Other, % of Pretax Earnings	0.50%	(0.60%)	0.60%
Total provision (benefit) for income taxes-continuing operations, % of Pretax Earnings	(13.50%)	2.10%	20.50%