Employee Benefit Plans - Expected Benefit Payments (Details) $ in Millions	Dec. 31, 2017 USD ($)
Expected benefit payments
Expected benefit payments, years 2023-2027	$ 27.4
Minimum
Expected benefit payments
Expected benefit payments, year one	2.9
Expected benefit payments, year two	2.9
Expected benefit payments, year three	2.9
Expected benefit payments, year four	2.9
Expected benefit payments, year five	2.9
Maximum
Expected benefit payments
Expected benefit payments, year one	4.8
Expected benefit payments, year two	4.8
Expected benefit payments, year three	4.8
Expected benefit payments, year four	4.8
Expected benefit payments, year five	$ 4.8