Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Jun. 30, 2020	Dec. 31, 2019
Statement of Financial Position [Abstract]
Amortized cost, current	$ 530.3	$ 557.3
Amortized cost, noncurrent	$ 193.5	$ 299.3
Preferred stock, par value (in USD per share)	$ 0.001	$ 0.001
Preferred stock, shares authorized (in shares)	5,000,000.0	5,000,000.0
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Common stock, par value (in USD per share)	$ 0.001	$ 0.001
Common stock, shares authorized (in shares)	220,000,000.0	220,000,000.0
Common stock, shares issued (in shares)	93,200,000	92,300,000
Common stock, shares outstanding (in shares)	93,200,000	92,300,000