Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015
Statement of Cash Flows [Abstract]
Net cash provided by Operating Activities	$ 417,307	$ 389,472
Cash flows from Investing Activities:
Additions to property and equipment, including pre-delivery payments	(104,250)	(105,329)
Proceeds from purchase assignment and leaseback transactions	31,851	86,033
Proceeds from disposition of equipment	0	3,606
Purchases of investments	(217,964)	(178,177)
Sales of investments	208,075	170,904
Net cash used in investing activities	(82,288)	(22,963)
Cash flows from Financing Activities:
Repayments of long-term debt and capital lease obligations	(205,532)	(74,719)
Repurchases and redemptions of convertible notes	(1,426)	(171,598)
Repurchases of common stock	(13,763)	(37,622)
Excess tax benefit from equity awards	17,615	0
Other	(7,702)	(1,483)
Net cash used in financing activities	(210,808)	(285,422)
Net increase in cash and cash equivalents	124,211	81,087
Cash and cash equivalents - Beginning of Period	281,502	264,087
Cash and cash equivalents - End of Period	$ 405,713	$ 345,174