Schedule II: Condensed Combined Financial Information of Registrant - Statement of Cash Flows (Details) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities:
Net loss	$ (133,300,000)	$ (85,200,000)	$ (38,900,000)	$ (105,600,000)	$ (85,200,000)	$ (100,200,000)	$ (47,300,000)	$ (49,700,000)	$ (363,000,000.0)	$ (282,400,000)	$ (69,100,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Share-based compensation									3,700,000	1,400,000	100,000
Tender offer									25,100,000	8,600,000	0
Depreciation and amortization, net									15,600,000	4,900,000	600,000
SAFE fair value adjustment									0	11,200,000	0
Warrants fair value adjustment									54,700,000	0	0
Paid-in kind interest expense									9,100,000	800,000	0
Changes in operating assets and liabilities:
Other assets									(21,700,000)	(3,700,000)	(2,700,000)
Accounts payable and accrued expenses									18,200,000	19,200,000	9,000,000.0
Other liabilities									2,100,000	2,700,000	(300,000)
Net cash used in operating activities									(287,200,000)	(127,200,000)	(26,100,000)
Cash flows from investing activities:
Purchases of investments									(158,400,000)	(138,100,000)	(40,300,000)
Proceeds from maturities, call and pay downs of investments									42,500,000	36,200,000	19,200,000
Capitalization of internally developed software									(5,400,000)	(5,500,000)	(2,300,000)
Purchases of fixed assets									(1,800,000)	(6,600,000)	(1,200,000)
Net cash used in investing activities									(114,100,000)	(114,000,000.0)	(20,600,000)
Cash flows from financing activities:
Proceeds from issuance of common stock from IPO and concurrent private placements, net of issuance costs									1,098,100,000	0	0
Proceeds from issuance of preferred stock, net of issuance costs									0	349,600,000	150,800,000
Proceeds from debt and warrants issuance, net of issuance costs									12,000,000.0	189,500,000	0
Repayments of long-term debt									(13,500,000)	(15,500,000)	0
Proceeds from SAFE									0	10,000,000.0	0
Purchases of treasury stock									(200,000)	0	0
Net cash provided by financing activities									1,098,500,000	535,500,000	150,900,000
Net increase in cash, cash equivalents and restricted cash									697,200,000	294,300,000	104,200,000
Cash, cash equivalents and restricted cash at beginning of year				416,600,000				122,300,000	416,600,000	122,300,000	18,100,000
Cash, cash equivalents and restricted cash at end of year	1,113,800,000				416,600,000				1,113,800,000	416,600,000	122,300,000
Supplemental disclosures:
Interest paid									4,500,000	4,300,000	800,000
Federal income taxes paid									0	0	0
Leasehold improvements - non-cash									0	1,500,000	2,600,000
Conversion of SAFE to preferred stock - noncash										11,200,000
Conversion of preferred stock to common stock - non-cash									560,400,000	0	0
Conversion of warrants to common stock - non-cash									75,000,000.0	0	0
Purchases of treasury stock - non-cash									500,000	0	0
Parent Company
Cash flows from operating activities:
Net loss									(363,000,000.0)	(282,400,000)	(69,100,000)
Adjustments to reconcile net loss to net cash used in operating activities:
Share-based compensation									3,700,000	1,400,000	100,000
Tender offer									25,100,000	8,600,000	0
Depreciation and amortization, net									11,800,000	3,500,000	200,000
Change in equity in subsidiaries									228,900,000	242,500,000	68,200,000
SAFE fair value adjustment									0	11,200,000	0
Warrants fair value adjustment									54,700,000	0	0
Paid-in kind interest expense									9,100,000	800,000	0
Changes in operating assets and liabilities:
Other assets									(15,500,000)	(1,200,000)	(1,100,000)
Accounts payable and accrued expenses									3,800,000	4,900,000	(400,000)
Other liabilities									2,100,000	600,000	300,000
Intercompany, net									(23,600,000)	(7,200,000)	(11,000,000.0)
Net cash used in operating activities									(62,900,000)	(17,300,000)	(12,800,000)
Cash flows from investing activities:
Purchases of investments									(500,000)	0	(9,800,000)
Proceeds from maturities, call and pay downs of investments									0	1,700,000	8,200,000
Capitalization of internally developed software									(5,400,000)	(3,900,000)	0
Purchases of fixed assets									(1,800,000)	(6,500,000)	(1,000,000.0)
Investment in subsidiaries									(201,900,000)	(333,000,000.0)	(95,000,000.0)
Net cash used in investing activities									(209,600,000)	(341,700,000)	(97,600,000)
Cash flows from financing activities:
Proceeds from issuance of common stock from IPO and concurrent private placements, net of issuance costs									1,098,100,000	0	0
Proceeds from exercise of stock options									2,100,000	1,900,000	100,000
Proceeds from issuance of preferred stock, net of issuance costs									0	349,600,000	150,800,000
Proceeds from debt and warrants issuance, net of issuance costs									12,000,000.0	189,500,000	0
Repayments of long-term debt									(13,500,000)	(15,500,000)	0
Proceeds from SAFE									0	10,000,000.0	0
Purchases of treasury stock									(200,000)	0	0
Net cash provided by financing activities									1,098,500,000	535,500,000	150,900,000
Net increase in cash, cash equivalents and restricted cash									826,000,000.0	176,500,000	40,500,000
Cash, cash equivalents and restricted cash at beginning of year				$ 218,100,000				$ 41,600,000	218,100,000	41,600,000	1,100,000
Cash, cash equivalents and restricted cash at end of year	$ 1,044,100,000				$ 218,100,000				1,044,100,000	218,100,000	41,600,000
Supplemental disclosures:
Interest paid									4,500,000	4,300,000	800,000
Federal income taxes paid									0	0	0
Leasehold improvements - non-cash									0	1,500,000	2,600,000
Conversion of SAFE to preferred stock - noncash									0	11,200,000	0
Conversion of preferred stock to common stock - non-cash									560,400,000	0	0
Conversion of warrants to common stock - non-cash									75,000,000.0	0	0
Purchases of treasury stock - non-cash									$ 500,000	$ 0	$ 0