Borrowing Arrangements (Details) - Revolving line of credit $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($)
Bank line of credit
Maximum borrowing capacity under the bank revolving line of credit	$ 100.0
Maximum letter of credit sublimit under the bank revolving line of credit	$ 25.0
Expiration date of the bank revolving line of credit	Nov. 03, 2020
Available borrowing capacity under the bank revolving line of credit, due to outstanding letters of credit	$ 99.0
Outstanding amount of letters of credit sublimit under the bank revolving line of credit	1.0
Available amount of letters of credit sublimit under the bank revolving line of credit	24.0
Borrowings outstanding under the bank revolving line of credit	0.0
Fair value of bonds pledged as collateral on the bank revolving line of credit	$ 110.3