Employee benefit plans - Plan asset allocation (Details)	Dec. 31, 2018
Employee benefit plans
Weighted average asset allocations (as a percent)	100.00%
Cash and cash equivalent
Employee benefit plans
Weighted average asset allocations (as a percent)	1.00%
Cash and cash equivalent | Minimum
Employee benefit plans
Weighted average asset allocations (as a percent)	0.00%
Cash and cash equivalent | Maximum
Employee benefit plans
Weighted average asset allocations (as a percent)	16.00%
U.S. equity securities
Employee benefit plans
Weighted average asset allocations (as a percent)	29.00%
U.S. equity securities | Minimum
Employee benefit plans
Weighted average asset allocations (as a percent)	15.00%
U.S. equity securities | Maximum
Employee benefit plans
Weighted average asset allocations (as a percent)	50.00%
International equity securities
Employee benefit plans
Weighted average asset allocations (as a percent)	31.00%
International equity securities | Minimum
Employee benefit plans
Weighted average asset allocations (as a percent)	9.00%
International equity securities | Maximum
Employee benefit plans
Weighted average asset allocations (as a percent)	33.00%
Fixed income
Employee benefit plans
Weighted average asset allocations (as a percent)	37.00%
Fixed income | Minimum
Employee benefit plans
Weighted average asset allocations (as a percent)	30.00%
Fixed income | Maximum
Employee benefit plans
Weighted average asset allocations (as a percent)	52.00%
Other
Employee benefit plans
Weighted average asset allocations (as a percent)	2.00%
Other | Minimum
Employee benefit plans
Weighted average asset allocations (as a percent)	0.00%
Other | Maximum
Employee benefit plans
Weighted average asset allocations (as a percent)	14.00%