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Postretirement Benefits Fair Values of Trust's Pension Assets (Details) - USD ($) $ in Millions | 12 Months Ended
--- | ---
Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Defined Contribution Plan Expense |  | $ 44.1 | $ 45.3 | $ 42.2
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets |  |  | 6.1 |
Cash and Cash Equivalents [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets |  |  | 0.0 |
Government Fixed-Income Securities [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets | [1],[2] |  | 0.0 |
Government Fixed-Income Securities [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets | [1],[2] |  | 126.2 |
Corporate Fixed-Income Securities [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets | [1],[3] |  | 0.0 |
Corporate Fixed-Income Securities [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets | [1],[3] |  | 356.2 |
Other Investment Companies [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets | [1],[4] |  | 0.0 |
Other Investment Companies [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets | [1],[4] |  | 7.2 |
Total Investments at Fair Value [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets | [1] |  | 6.1 |
Total Investments at Fair Value [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets | [1] |  | 489.6 |
Other Pension Plan Liabilities [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets | [5] |  | (21.3) |
Total Pension Plan Net Assets [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets |  |  | 642.0 |
Total Pension Plan Net Assets [Member] | Fair Value, Inputs, Level 3 [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets |  | $ 0.0 |  |
Estimate of Fair Value Measurement [Member] | Cash and Cash Equivalents [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets |  |  | 6.1 |
Estimate of Fair Value Measurement [Member] | Government Fixed-Income Securities [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets | [2] |  | 126.2 |
Estimate of Fair Value Measurement [Member] | Corporate Fixed-Income Securities [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets | [3] |  | 356.2 |
Estimate of Fair Value Measurement [Member] | Other Investment Companies [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets | [4] |  | 7.2 |
Estimate of Fair Value Measurement [Member] | Total Investments at Fair Value [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets |  |  | 495.7 |
Portion at Other than Fair Value Measurement [Member] | Short-Term Investments [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets |  |  | 102.3 |
Portion at Other than Fair Value Measurement [Member] | Equity Securities [Member] | United States [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets | [6] |  | 2.0 |
Portion at Other than Fair Value Measurement [Member] | Commingled Funds [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets | [7] |  | 63.3 |
Portion at Other than Fair Value Measurement [Member] | Total Investments at Net Asset Value [Member] |  |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |  |
Total Pension Plan Net Assets |  |  | $ 167.6 |
[1] | (A) See Note 7 – Fair Value Measurements for a description of levels within the fair value hierarchy. The level in the fair value hierarchy within which the fair value measurement is classified is determined based on the lowest level input that is significant to the fair value measurement in its entirety. A description of the valuation methodologies is provided following these tables. There were no transfers in and/or out of Level 1 and Level 2 in 2018.
[2] | (C) Government securities are comprised of U.S. Treasury bonds and other government securities.
[3] | (D) Corporate securities consist primarily of a diversified portfolio of investment grade bonds issued by companies.
[4] | (F) Other investments consist primarily of interest rate swaps used to manage the average duration of the fixed income portfolio and credit default swaps to manage credit risk exposure.
[5] | (G) This class includes interest receivable and receivables/payables for securities sold/purchased.
[6] | (B) The equity assets are invested in two indexed funds based on the Russell 3000 Index (U.S.) and the MSCI EAFE Equity Index (International). The Trust did not directly own any of the Company's common stock as of December 31, 2018.
[7] | (E) This class includes commingled funds that primarily invest in investment grade corporate securities and government-related securities. This class also includes investments in non-agency collateralized mortgage obligations and mortgage-backed securities, futures and options.

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