Employee Benefit Plans - Plan Assets Measured at Fair Value (Detail) - Pension Plans [Member] - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	$ 746.7	$ 596.4	$ 584.9
Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	746.7	596.4
Cash Equivalents [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	18.9	15.2
Equity Common Stocks [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	261.2	216.6
Equity Mutual Funds [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	215.2	201.1
Corporate [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	73.0	68.5
Mutual Funds [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	72.5	32.9
U.S. Treasury and Agency [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	104.8	58.6
Other [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	1.1	3.5
Level 1 [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	280.1	231.8
Level 1 [Member] | Cash Equivalents [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	18.9	15.2
Level 1 [Member] | Equity Common Stocks [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	261.2	216.6
Level 1 [Member] | Equity Mutual Funds [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0.0	0.0
Level 1 [Member] | Corporate [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0.0	0.0
Level 1 [Member] | Mutual Funds [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0.0	0.0
Level 1 [Member] | U.S. Treasury and Agency [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0.0	0.0
Level 1 [Member] | Other [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0.0	0.0
Level 2 [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	466.6	364.6
Level 2 [Member] | Cash Equivalents [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0.0	0.0
Level 2 [Member] | Equity Common Stocks [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0.0	0.0
Level 2 [Member] | Equity Mutual Funds [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	215.2	201.1
Level 2 [Member] | Corporate [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	73.0	68.5
Level 2 [Member] | Mutual Funds [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	72.5	32.9
Level 2 [Member] | U.S. Treasury and Agency [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	104.8	58.6
Level 2 [Member] | Other [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	1.1	3.5
Level 3 [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0.0	0.0
Level 3 [Member] | Cash Equivalents [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0.0	0.0
Level 3 [Member] | Equity Common Stocks [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0.0	0.0
Level 3 [Member] | Equity Mutual Funds [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0.0	0.0
Level 3 [Member] | Corporate [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0.0	0.0
Level 3 [Member] | Mutual Funds [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0.0	0.0
Level 3 [Member] | U.S. Treasury and Agency [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	0.0	0.0
Level 3 [Member] | Other [Member] | Qualified Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension plan assets measured at fair value	$ 0.0	$ 0.0