Condensed Consolidated Statements of Cash Flows - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014
Statement of Cash Flows [Abstract]
Net cash provided by Operating Activities	$ 297,520	$ 203,969
Cash flows from Investing Activities:
Additions to property and equipment, including pre-delivery payments	(72,902)	(331,766)
Proceeds from purchase assignment and leaseback transactions	86,033	0
Proceeds from disposition of equipment	3,513	350
Purchases of investments	(115,218)	(234,143)
Sales of investments	112,886	30,859
Net cash provided by (used in) investing activities	14,312	(534,700)
Cash flows from Financing Activities:
Long-term borrowings	0	293,430
Repayments of long-term debt and capital lease obligations	(43,460)	(30,756)
Repurchases of convertible notes	(168,407)	0
Repurchases of common stock	(17,621)	0
Other	(1,649)	4,954
Net cash provided by (used in) financing activities	(231,137)	267,628
Net increase (decrease) in cash and cash equivalents	80,695	(63,103)
Cash and cash equivalents - Beginning of Period	264,087	423,384
Cash and cash equivalents - End of Period	$ 344,782	$ 360,281