Condensed Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Statement Of Financial Position [Abstract]
Amortized cost, current	$ 584.1	$ 557.3
Amortized cost, noncurrent	$ 237.8	$ 299.3
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000.0	5,000,000.0
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	220,000,000.0	220,000,000.0
Common stock, shares issued	92,800,000	92,300,000
Common stock, shares outstanding	92,800,000	92,300,000