Investments Continuous Unrealized Loss Position (Details) $ in Millions	Mar. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in a continuous unrealized loss position, less than twelve months	$ 385.0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(0.4)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0.0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0.0
Fair value of available-for-sale securities in a continuous unrealized loss position	385.0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(0.4)
Government agency bonds
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in a continuous unrealized loss position, less than twelve months	196.9
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(0.2)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0.0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0.0
Fair value of available-for-sale securities in a continuous unrealized loss position	196.9
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	(0.2)
Municipal bonds - tax exempt
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in a continuous unrealized loss position, less than twelve months	55.3
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	0.0
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0.0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0.0
Fair value of available-for-sale securities in a continuous unrealized loss position	55.3
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	0.0
Corporate bonds and debt
Schedule of Available-for-sale Securities [Line Items]
Fair value of available-for-sale securities in a continuous unrealized loss position, less than twelve months	132.8
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, less than twelve months	(0.2)
Fair value of available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0.0
Accumulated loss, available-for-sale securities in a continuous unrealized loss position, greater than twelve months	0.0
Fair value of available-for-sale securities in a continuous unrealized loss position	132.8
Accumulated loss, available-for-sale securities in a continuous unrealized loss position	$ (0.2)