Income Taxes - Unrecognized Income Tax Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Balance at beginning of year	$ 5.2	$ 2.7	$ 2.8
Additions for tax positions taken in prior years	0.5	2.5	0.1
Reductions attributable to audit settlements/lapse of statute of limitations	0.0	0.0	(0.2)
Balance at end of year	33.3	5.2	2.7
United Financial [Member] | D.C. Solar investment [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Additions related to D.C. Solar	8.7	0.0	0.0
People's United Financial, Inc. [Member] | D.C. Solar investment [Member]
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Additions related to D.C. Solar	$ 18.9	$ 0.0	$ 0.0