Other Comprehensive Income (Loss) (Components of Other Comprehensive Income (Loss)) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Components of Other Comprehensive Income (Loss) [Line Items]
Other comprehensive income (loss), gross total	$ (9,042)	$ 875	$ (12,293)	$ (120)
Income tax (expense) benefit related to items of other comprehensive income (loss)	3,403	(336)	4,650	49
Amounts reclassified, net of tax			3,270	1,496
Changes in fair value, net of tax			(10,913)	(1,567)
Other comprehensive income (loss), total, net of tax	(5,639)	539	(7,643)	(71)
Interest Rate Swap [Member]
Components of Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified, gross	1,041	1,313	3,114	3,267
Changes in fair value, gross	(6,066)	968	(10,503)	(1,536)
Amounts reclassified, tax effect	(396)	(504)	(1,185)	(1,253)
Changes in fair value, tax effect	2,280	(371)	3,975	594
Amounts reclassified, net of tax	645	809	1,929	2,014
Changes in fair value, net of tax	(3,786)	597	(6,528)	(942)
Fuel [Member] | Commodity Contract [Member]
Components of Other Comprehensive Income (Loss) [Line Items]
Amounts reclassified, gross	874	(214)	2,165	(841)
Changes in fair value, gross	(4,891)	(1,192)	(7,069)	(1,010)
Amounts reclassified, tax effect	(333)	82	(824)	323
Changes in fair value, tax effect	1,852	457	2,684	385
Amounts reclassified, net of tax	541	(132)	1,341	(518)
Changes in fair value, net of tax	$ (3,039)	$ (735)	$ (4,385)	$ (625)