Supplemental Guarantor Financial Information - I/S (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Condensed Consolidating Statements of Comprehensive Income
Ownership, as a percent	100.00%		100.00%
Revenues	$ 328,334	$ 311,350	$ 955,542	$ 896,997
Operating costs and expenses	284,967	268,118	843,776	790,169
Income from operations	43,367	43,232	111,766	106,828
Interest expense (income) net, including amortization of deferred financing costs and note discount	7,141	7,892	21,863	22,951
Other (income) expense	360	1,067	748	2,882
Income before income taxes	35,866	34,273	89,155	80,995
Income tax (benefit) expense	8,388	12,629	26,204	29,837
Net income	27,478	21,644	62,951	51,158
Net loss attributable to noncontrolling interests	(12)	(365)	(71)	(1,081)
Net income attributable to controlling interests and available to common stockholders	27,490	22,009	63,022	52,239
Other comprehensive income (loss), attributable to controlling interest	1,497	(22,554)	(38,425)	(9,064)
Comprehensive income (loss) attributable to controlling interests	28,987	(545)	24,597	43,175
Consolidation Eliminations
Condensed Consolidating Statements of Comprehensive Income
Revenues	(6,503)	(3,007)	(18,800)	(8,313)
Operating costs and expenses	(6,503)	(3,007)	(18,800)	(8,313)
Equity in earnings of subsidiaries	65,389	38,525	171,243	79,461
Other (income) expense	(32)		(47)
Income before income taxes	(65,357)	(38,525)	(171,196)	(79,461)
Net income	(65,357)	(38,525)	(171,196)	(79,461)
Net loss attributable to noncontrolling interests	(12)	(365)	(71)	(1,081)
Net income attributable to controlling interests and available to common stockholders	(65,345)	(38,160)	(171,125)	(78,380)
Other comprehensive income (loss), attributable to controlling interest	60,334	20,839	19,721	1,124
Comprehensive income (loss) attributable to controlling interests	(5,011)	(17,321)	(151,404)	(77,256)
Parent
Condensed Consolidating Statements of Comprehensive Income
Operating costs and expenses	9,085		9,085
Income from operations	(9,085)		(9,085)
Equity in earnings of subsidiaries	(35,950)		(71,423)
Other (income) expense	(4)		(4)
Income before income taxes	26,869		62,342
Income tax (benefit) expense	(609)		(609)
Net income	27,478		62,951
Net income attributable to controlling interests and available to common stockholders	27,478		62,951
Other comprehensive income (loss), attributable to controlling interest	1,510		(38,353)
Comprehensive income (loss) attributable to controlling interests	28,988		24,598
Issuer
Condensed Consolidating Statements of Comprehensive Income
Operating costs and expenses	336	4,862	14,394	14,062
Income from operations	(336)	(4,862)	(14,394)	(14,062)
Interest expense (income) net, including amortization of deferred financing costs and note discount	6,306	6,042	19,153	16,327
Equity in earnings of subsidiaries	(18,890)	(31,261)	(70,923)	(68,831)
Other (income) expense	69	(1,235)	(5)	(313)
Income before income taxes	12,179	21,592	37,381	38,755
Income tax (benefit) expense	(6,686)	(52)	(17,151)	(12,403)
Net income	18,865	21,644	54,532	51,158
Net income attributable to controlling interests and available to common stockholders	18,865	21,644	54,532	51,158
Other comprehensive income (loss), attributable to controlling interest	(16,012)	(22,555)	(4,744)	(9,063)
Comprehensive income (loss) attributable to controlling interests	2,853	(911)	49,788	42,095
Guarantors
Condensed Consolidating Statements of Comprehensive Income
Revenues	229,126	205,150	663,998	590,533
Operating costs and expenses	193,789	172,564	572,241	497,229
Income from operations	35,337	32,586	91,757	93,304
Interest expense (income) net, including amortization of deferred financing costs and note discount	14,818	1,470	15,605	4,863
Equity in earnings of subsidiaries	(10,549)	(7,264)	(28,897)	(10,630)
Other (income) expense	(598)	(963)	(2,622)	(2,594)
Income before income taxes	31,666	39,343	107,671	101,665
Income tax (benefit) expense	12,580	9,891	36,690	37,706
Net income	19,086	29,452	70,981	63,959
Net income attributable to controlling interests and available to common stockholders	19,086	29,452	70,981	63,959
Other comprehensive income (loss), attributable to controlling interest	(16,690)	(7,158)	(9,215)	1,300
Comprehensive income (loss) attributable to controlling interests	2,396	22,294	61,766	65,259
Non-Guarantors
Condensed Consolidating Statements of Comprehensive Income
Revenues	105,711	109,207	310,344	314,777
Operating costs and expenses	88,260	93,699	266,856	287,191
Income from operations	17,451	15,508	43,488	27,586
Interest expense (income) net, including amortization of deferred financing costs and note discount	(13,983)	380	(12,895)	1,761
Other (income) expense	925	3,265	3,426	5,789
Income before income taxes	30,509	11,863	52,957	20,036
Income tax (benefit) expense	3,103	2,790	7,274	4,534
Net income	27,406	9,073	45,683	15,502
Net income attributable to controlling interests and available to common stockholders	27,406	9,073	45,683	15,502
Other comprehensive income (loss), attributable to controlling interest	(27,645)	(13,680)	(5,834)	(2,425)
Comprehensive income (loss) attributable to controlling interests	$ (239)	$ (4,607)	$ 39,849	$ 13,077