Schedule I Condensed Financial Information of Registrant Parent Company Only - Cash Flows (Details) - USD ($) $ in Thousands			3 Months Ended									12 Months Ended
	Nov. 19, 2019	Feb. 05, 2018	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2019	Dec. 31, 2018
Cash flows from operating activities
Net income			$ 98,631	$ 144,435	$ 152,049	$ 150,849	$ 142,541	$ 129,912	$ 139,044	$ 112,122	$ 42,662	$ 545,964	$ 523,619
Change in operating assets and liabilities:
Change in other assets											(7,159)	(5,635)	(22,945)
Change in other liabilities											10,449	339	25,207
Net cash provided by operating activities											129,440	682,159	504,082
Cash flows from investing activities
Investments in short-term investments											0	(440,353)	(942,311)
Maturities of short-term investments											0	901,756	421,434
Net cash used in investing activities											(1,136,251)	(1,361,379)	(1,140,877)
Cash flows from financing activities
Proceeds from follow-on offering of common stock	$ 694,200	$ 1,300,000
Proceeds from private placement of common stock											963,782	0	0
Proceeds from initial public offering of common stock											0	1,164,307	2,001,493
Dividends paid											0	(503,958)	(262,682)
Mandatory debt conversion costs											(13)	0	0
Net cash provided by financing activities											1,148,446	1,182,666	1,037,836
Net increase in cash, cash equivalents and restricted cash											141,635	503,446	401,041
Cash and cash equivalents, beginning of period						598,447				197,406	55,771	598,447	197,406
Cash, cash equivalents and restricted cash, end of period			1,101,893				598,447				197,406	1,101,893	598,447
Parent Company
Cash flows from operating activities
Net income											282	545,964	523,619
Income (Loss) from Subsidiaries, before Tax											0	(532,699)	0
Distributed Earnings											0	13,334	0
Change in operating assets and liabilities:
Change in other assets											0	48	(2,150)
Change in other liabilities											0	1,370	270
Change in intercompany balances, net											98,813	(1,985)	(614)
Net cash provided by operating activities											99,095	26,032	521,125
Cash flows from investing activities
Investment in subsidiary											(1,000,000)	(1,700,748)	(1,838,205)
Distributions from subsidiaries											0	232,875	357,781
Investments in short-term investments											0	(342,767)	(691,239)
Maturities of short-term investments											0	760,419	170,362
Net cash used in investing activities											(1,000,000)	(1,050,221)	(2,001,301)
Cash flows from financing activities
Proceeds from follow-on offering of common stock											0	1,164,307	694,374
Proceeds from private placement of common stock											964,376	0	0
Proceeds from initial public offering of common stock											0	0	1,307,119
Dividends paid											0	(503,958)	(262,682)
Mandatory debt conversion costs											(13)	0	0
Net cash provided by financing activities											964,363	660,349	1,738,811
Net increase in cash, cash equivalents and restricted cash											63,458	(363,840)	258,635
Cash and cash equivalents, beginning of period						$ 377,752				$ 119,117	55,659	377,752	119,117
Cash, cash equivalents and restricted cash, end of period			$ 13,912				$ 377,752				$ 119,117	$ 13,912	$ 377,752