Benefit Plans - Pension plan assets narrative (Details) - Pension benefits - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension plans' assets
Investments measured using the net asset value practical expedient	$ 2,687	$ 3,598
Percentage of total plan assets for primary qualified employee	81.00%
Notional amount of derivatives, minimum percentage of plan assets	105.00%
Notional amount of derivatives, maximum percentage of plan assets	115.00%
Expected weighted average long-term rate of return on plan assets	7.32%	7.31%	7.30%
Expected weighted average long-term rate of return on plan assets for the next fiscal year	7.33%
Expected weighted average long-term rate of return on primary employee plan assets for the next fiscal year	7.75%
Expected weighted average long-term rate of return on employee-agent plan assets for the next fiscal year	5.75%
Arithmetic average of the annual actual return on plan assets for the last 10 years	9.00%
Arithmetic average of the annual actual return on plan assets for the last 5 years	6.60%
Equity securities
Pension plans' assets
Percent of total equity securities in primary qualified employee plan	82.00%