Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Statement of Cash Flows [Abstract]
Net income	$ 208,288	$ 129,843
Adjustments to reconcile net income to net cash provided by operating activities:
Depreciation and amortization of property, equipment, and intangibles	161,210	131,664
Gain on sale of property and equipment	(34,801)	(10,537)
Impairments	810	0
Deferred income taxes	4,246	(18,920)
Non-cash lease expense	9,490	15,589
Other adjustments to reconcile net income to net cash provided by operating activities	32,229	(6,522)
Increase (decrease) in cash resulting from changes in:
Trade receivables	(28,007)	(11,586)
Income tax receivable	692	2,872
Accounts payable	22,074	12,534
Accrued liabilities and claims accrual	89,289	70,975
Operating lease liabilities	(9,267)	(15,174)
Other assets and liabilities	607	5,375
Net cash provided by operating activities	456,860	306,113
Cash flows from investing activities:
Proceeds from maturities of held-to-maturity investments	1,881	500
Purchases of held-to-maturity investments	(4,372)	(512)
Proceeds from sale of property and equipment, including assets held for sale	60,532	67,175
Purchases of property and equipment	(164,974)	(111,020)
Expenditures on assets held for sale	(43)	(401)
Net cash, restricted cash, and equivalents invested in acquisitions	(1,291)	(39,281)
Other cash flows from investing activities	(1,920)	9,398
Net cash used in investing activities	(110,187)	(74,141)
Cash flows from financing activities:
Repayment of finance leases and long-term debt	(48,843)	(6,600)
Repayments on revolving lines of credit, net	(95,000)	(95,000)
Repayment of accounts receivable securitization	0	(15,000)
Proceeds from common stock issued	1,220	2,709
Repurchases of the Company's common stock	(144,881)	(53,661)
Dividends paid	(20,137)	(13,624)
Other cash flows from financing activities	(15,608)	(4,190)
Net cash used in financing activities	(323,249)	(185,366)
Net increase in cash, restricted cash, and equivalents	23,424	46,606
Cash, restricted cash, and equivalents at beginning of period	350,023	197,277
Cash, restricted cash, and equivalents at end of period	373,447	243,883
Cash paid during the period for:
Interest	5,928	2,505
Income taxes	1,778	2,199
Other Significant Noncash Transactions [Line Items]
Equipment acquired included in accounts payable	11,643	13,860
Purchase price adjustment on acquisition	2,163	0
Right-of-use assets obtained (forfeited) in exchange for operating lease liabilities	3,314	(2,608)
Financing provided to independent contractors for equipment sold [Member]
Other Significant Noncash Transactions [Line Items]
Other non-cash investing and financing activities	1,536	462
Transfers from property and equipment to assets held for sale [Member]
Other Significant Noncash Transactions [Line Items]
Other non-cash investing and financing activities	16,986	29,955
Noncontrolling Interest Associated With Acquisition
Other Significant Noncash Transactions [Line Items]
Other non-cash investing and financing activities	0	10,281
Property and equipment obtained in exchange for finance lease liabilities from operating lease liabilities
Other Significant Noncash Transactions [Line Items]
Property and equipment obtained in exchange for finance lease liabilities reclassified from operating lease liabilities	0	28,149
Acquisitions [Member]
Other Significant Noncash Transactions [Line Items]
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability	$ 0
2021 Acquisitions
Other Significant Noncash Transactions [Line Items]
Right-of-Use Asset Obtained in Exchange for Operating Lease Liability		$ 560