Derivative Instruments, Foreign Exchange Contracts (Details) - Foreign Exchange Contract [Member] ¥ in Millions, $ in Millions	6 Months Ended
	Jun. 30, 2018 AUD ($)	Jun. 30, 2018 JPY (¥)	Dec. 31, 2017 AUD ($)	Dec. 31, 2017 EUR (€)	Dec. 31, 2017 JPY (¥)
Derivative [Abstract]
Foreign exchange contract outstanding	$ 0.2	¥ 112.5	$ 0.2	€ 24,000	¥ 162.5
Expiration date of contract	Dec. 31, 2018