Guarantees (Details 5)	Dec. 31, 2017 USD ($)
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure, undiscounted	$ 108,000,000.0
Fixed maximum amounts guaranteed by year 2018	6,400,000
Fixed maximum amounts guaranteed by year 2019	25,900,000
Fixed maximum amounts guaranteed by year 2020	68,700,000
Fixed maximum amounts guaranteed by year 2021	500,000
Fixed maximum amounts guaranteed by year 2022	500,000
Fixed maximum amounts guaranteed, thereafter	2,000,000.0
Fixed maximum amounts guaranteed by year no maturity date	4,000,000.0
Amount outstanding under guarantees	0
Letters of credit	34,000,000.0
Outstanding letters of credit	0
Amount of surety bonds outstanding	616,500,000
Financial guarantee [Member]
Guarantor Obligations [Line Items]
Guarantor obligations, maximum exposure, undiscounted	$ 56,300,000