Description of the Business and Significant Accounting Policies (Earnings Per Share) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive securities excluded from computation of earnings per share amount
Antidilutive common share equivalents excluded from computation of earnings per share amount (in shares)									0	0	0
Net Income (Numerator)
Basic earnings per share - net income (in dollars)	$ 270,004	$ 398,375	$ 362,449	$ 227,551	$ 304,733	$ 153,258	$ 153,933	$ 200,817	$ 1,258,379	$ 812,741	$ 382,115
Diluted earnings per share - net income (in dollars)									$ 1,258,379	$ 812,741	$ 382,115
Weighted average common shares outstanding (in shares)
Weighted average common shares outstanding - basic (in shares)									233,923,000	240,132,000	243,576,000
Dilutive common share equivalents (in shares)									1,270,000	1,649,000	1,722,000
Weighted average common shares outstanding - diluted (in shares)									235,193,000	241,781,000	245,298,000
Per Share Amount
Basic (in dollars per share)	$ 1.18	$ 1.70	$ 1.54	$ 0.96	$ 1.28	$ 0.64	$ 0.64	$ 0.83	$ 5.38	$ 3.38	$ 1.57
Diluted (in dollars per share)	$ 1.17	$ 1.69	$ 1.53	$ 0.96	$ 1.28	$ 0.64	$ 0.63	$ 0.82	$ 5.35	$ 3.36	$ 1.56
Parent Company [Member]
Net Income (Numerator)
Basic earnings per share - net income (in dollars)									$ 1,258,379	$ 812,741	$ 382,115
Guarantors
Net Income (Numerator)
Basic earnings per share - net income (in dollars)									725,559	399,357	299,318
Combined Non-Guarantors
Net Income (Numerator)
Basic earnings per share - net income (in dollars)									$ (15,496)	$ (10,975)	$ (108,250)