Employee Benefits - Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	$ 3,225.0	$ 3,007.0	$ 2,690.8
Pension Plan | Level 1, 2, and 3
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	1,955.0	1,778.1
Pension Plan | Level 1, 2, and 3 | United States equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	439.2	335.6
Pension Plan | Level 1, 2, and 3 | International equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	345.1	322.3
Pension Plan | Level 1, 2, and 3 | United States bonds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	1,056.4	981.7
Pension Plan | Level 1, 2, and 3 | International bonds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	114.3	138.5
Pension Plan | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	784.3	803.0
Pension Plan | Level 1 | United States equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	439.2	335.6
Pension Plan | Level 1 | International equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	345.1	321.6
Pension Plan | Level 1 | United States bonds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	94.3
Pension Plan | Level 1 | International bonds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	51.5
Pension Plan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	1,170.7	975.1
Pension Plan | Level 2 | United States equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	0.0
Pension Plan | Level 2 | International equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	0.7
Pension Plan | Level 2 | United States bonds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	1,056.4	887.4
Pension Plan | Level 2 | International bonds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	114.3	87.0
Pension Plan | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	0.0
Pension Plan | Level 3 | United States equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	0.0
Pension Plan | Level 3 | International equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	0.0
Pension Plan | Level 3 | United States bonds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	0.0
Pension Plan | Level 3 | International bonds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	0.0
Pension Plan | Investments measured at net asset value per share
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	1,270.0	1,228.9
OPEB Plan
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	951.4	879.6	$ 771.7
OPEB Plan | Level 1, 2, and 3
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	597.2	528.6
OPEB Plan | Level 1, 2, and 3 | United States equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	141.4	103.0
OPEB Plan | Level 1, 2, and 3 | International equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	120.9	107.5
OPEB Plan | Level 1, 2, and 3 | United States bonds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	322.9	285.0
OPEB Plan | Level 1, 2, and 3 | International bonds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	12.0	33.1
OPEB Plan | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	405.3	354.0
OPEB Plan | Level 1 | United States equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	141.4	103.0
OPEB Plan | Level 1 | International equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	120.9	107.3
OPEB Plan | Level 1 | United States bonds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	143.0	119.1
OPEB Plan | Level 1 | International bonds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	24.6
OPEB Plan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	191.9	174.6
OPEB Plan | Level 2 | United States equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	0.0
OPEB Plan | Level 2 | International equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	0.2
OPEB Plan | Level 2 | United States bonds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	179.9	165.9
OPEB Plan | Level 2 | International bonds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	12.0	8.5
OPEB Plan | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	0.0
OPEB Plan | Level 3 | United States equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	0.0
OPEB Plan | Level 3 | International equity
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	0.0
OPEB Plan | Level 3 | United States bonds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	0.0
OPEB Plan | Level 3 | International bonds
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	0.0	0.0
OPEB Plan | Investments measured at net asset value per share
Defined Benefit Plan Disclosure [Line Items]
Fair Value of Plan Assets	$ 354.2	$ 351.0