Dividend Reinvestment and Stock Purchase Plan (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended			46 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014
Dividend Reinvestment and Stock Purchase Plan
Authorized common shares to be issued	6,000,000
Number of common shares issued	3,527,166	1,449,139	55,598	5,091,508
Amount raised from shares issued	$ 158.5	$ 55.6	$ 2.2	$ 218.6
Number of common shares issued	3,330,556	1,308,490
Amount raised from shares issued	$ 150.0	$ 50.0