Employee benefit plans - Target Asset Allocations (Details)	Dec. 31, 2019
Target asset allocations for each major category of the plan assets
Actual	100.00%
Percentage points to rebalance the portfolio within	2.00%
Cash and cash equivalents
Target asset allocations for each major category of the plan assets
Target	0.00%
Actual	1.00%
U.S. equity securities
Target asset allocations for each major category of the plan assets
Target	26.00%
Actual	26.00%
Global equity securities
Target asset allocations for each major category of the plan assets
Target	36.00%
Actual	35.00%
Fixed income
Target asset allocations for each major category of the plan assets
Target	34.00%
Actual	34.00%
Other
Target asset allocations for each major category of the plan assets
Target	4.00%
Actual	4.00%