Selected Quarterly Financial Information - Schedule of Selected Quarterly Financial Information (unaudited) (Details) - USD ($)	3 Months Ended																12 Months Ended
	Dec. 31, 2019	[1]	Sep. 30, 2019	[1]	Jun. 30, 2019	[1]	Mar. 31, 2019	[1]	Dec. 31, 2018	[1]	Sep. 30, 2018	[1]	Jun. 30, 2018	[1]	Mar. 31, 2018	[1]	Dec. 31, 2019	[1]	Dec. 31, 2018	[1]	Dec. 31, 2017
Selected Quarterly Financial Information [Abstract]
Total revenues	$ 468,000,000		$ 466,000,000		$ 454,000,000		$ 450,000,000		$ 642,000,000		$ 427,000,000		$ 563,000,000		$ 367,000,000		$ 1,838,000,000		$ 1,999,000,000		$ 1,711,000,000
Total operating expenses	384,000,000		384,000,000		390,000,000		365,000,000		473,000,000		364,000,000		408,000,000		320,000,000		1,523,000,000		1,565,000,000		1,374,000,000
Income before income taxes	74,000,000		70,000,000		54,000,000		75,000,000		161,000,000		56,000,000		146,000,000		40,000,000		273,000,000		403,000,000		311,000,000
Net income	$ 72,000,000		$ 50,000,000		$ 39,000,000		$ 55,000,000		$ 120,000,000		$ 41,000,000		$ 107,000,000		$ 30,000,000		$ 215,695,961	[2]	$ 298,124,983	[2]	$ 326,777,744	[2]
Basic	$ 0.83		$ 0.59		$ 0.43		$ 0.59		$ 1.25		$ 0.42		$ 1.10		$ 0.31		$ 2.43		$ 3.07		$ 3.30
Diluted	$ 0.83		$ 0.59		$ 0.43		$ 0.58		$ 1.24		$ 0.42		$ 1.10		$ 0.30		$ 2.42		$ 3.05		$ 3.28

[1] The sum of the earnings per share for the four quarters may differ from annual earnings per share due to the required method of computing the weighted average shares in interim periods.

[2] Net income for years ended December 31, 2019, 2018, and 2017 was $215,695,961; $298,124,983 and $326,777,744, respectively.