Postretirement Benefits - Asset Allocation (Details) - Employee pension plan - Qualified Plan - Funded Plan	Dec. 31, 2019	Dec. 31, 2018
Target asset allocation for the portfolio:
Total plan assets (as a percent)	100.00%	100.00%
Actual asset allocation for the portfolio:
Total plan assets (as a percent)	100.00%	100.00%
Total equity securities
Target asset allocation for the portfolio:
Total plan assets (as a percent)	45.00%	41.00%
Actual asset allocation for the portfolio:
Total plan assets (as a percent)	46.00%	38.00%
U.S. equity securities
Target asset allocation for the portfolio:
Total plan assets (as a percent)	27.00%	25.00%
Actual asset allocation for the portfolio:
Total plan assets (as a percent)	28.00%	24.00%
U.S. small capitalization core equity
Target asset allocation for the portfolio:
Total plan assets (as a percent)	22.00%
U.S. equity index
Target asset allocation for the portfolio:
Total plan assets (as a percent)	78.00%
Non-U.S. equity securities
Target asset allocation for the portfolio:
Total plan assets (as a percent)	18.00%	16.00%
Actual asset allocation for the portfolio:
Total plan assets (as a percent)	18.00%	14.00%
International small capitalization risk controlled equity
Target asset allocation for the portfolio:
Total plan assets (as a percent)	11.00%
International emerging markets equity
Target asset allocation for the portfolio:
Total plan assets (as a percent)	20.00%
International risk controlled equity
Target asset allocation for the portfolio:
Total plan assets (as a percent)	69.00%
Debt securities
Target asset allocation for the portfolio:
Total plan assets (as a percent)	54.00%	58.00%
Actual asset allocation for the portfolio:
Total plan assets (as a percent)	53.00%	61.00%
U.S. Treasury Strips
Target asset allocation for the portfolio:
Total plan assets (as a percent)	33.00%
U.S. corporate bonds with an emphasis on long duration bonds rated A or better
Target asset allocation for the portfolio:
Total plan assets (as a percent)	67.00%
Other
Target asset allocation for the portfolio:
Total plan assets (as a percent)	1.00%	1.00%
Actual asset allocation for the portfolio:
Total plan assets (as a percent)	1.00%	1.00%