Postretirement Benefits - Asset Allocation (Details) - Employee pension plan - Qualified Plan - Funded Plan	Dec. 31, 2018	Dec. 31, 2017
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)	41.00%	55.00%
Actual asset allocation for the portfolio:
Total plan assets (as a percent)	38.00%	58.00%
U.S. equity securities
Target asset allocation for the portfolio:
Total plan assets (as a percent)	25.00%	35.00%
Actual asset allocation for the portfolio:
Total plan assets (as a percent)	24.00%	39.00%
U.S. small capitalization core equity
Target asset allocation for the portfolio:
Total plan assets (as a percent)	21.00%
U.S. equity index
Target asset allocation for the portfolio:
Total plan assets (as a percent)	79.00%
Non-U.S. equity securities
Target asset allocation for the portfolio:
Total plan assets (as a percent)	16.00%	20.00%
Actual asset allocation for the portfolio:
Total plan assets (as a percent)	14.00%	19.00%
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)	58.00%	44.00%
Actual asset allocation for the portfolio:
Total plan assets (as a percent)	61.00%	41.00%
U.S. Treasury Strips
Target asset allocation for the portfolio:
Total plan assets (as a percent)	32.00%
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)	62.00%
Floating rate high income leverage loans
Target asset allocation for the portfolio:
Total plan assets (as a percent)	6.00%
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%