Reconciliation of Net Income (Loss) to EBITDA (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
		Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting [Abstract]
Net income		$ 9,981	$ 22,007	$ 34,159	$ 49,854
Interest expense		7,135	5,729	19,364	16,801
Interest income		(60)	(73)	(236)	(221)
Change in fair value of interest rate swaps		(836)	0	765	0
Loss on extinguishment of debt		9,525	0	9,525	0
Income tax provision		5,522	12,629	19,188	28,839
Depreciation and amortization		19,459	14,249	53,249	41,117
EBITDA	[1]	$ 50,700	$ 54,500	$ 136,000	$ 136,400

[1] EBITDA is defined as income (loss) before interest (interest expense, interest income, change in fair value of interest rate swaps, and loss on extinguishment of debt), income taxes, and depreciation and amortization. EBITDA is the primary measure used by our chief operating decision maker to evaluate segment operating performance and to decide how to allocate resources to segments. We believe EBITDA is useful to investors because it provides a means to evaluate the operating performance of our segments and our company on an ongoing basis using criteria that are used by our internal decision makers and because it is frequently used by investors and other interested parties when comparing companies in our industry that have different financing and capital structures and/or tax rates. We believe EBITDA is a meaningful measure because it presents a transparent view of our recurring operating performance and allows management to readily view operating trends, perform analytical comparisons, and identify strategies to improve operating performance. EBITDA, however, is not a measure of our liquidity or financial performance under GAAP and should not be considered as an alternative to net income (loss), income (loss) from operations, or any other performance measure derived in accordance with GAAP or as an alternative to cash flow from operating activities as a measure of our liquidity. The use of EBITDA instead of net income (loss) or segment income (loss) has limitations as an analytical tool, including the inability to determine profitability; the exclusion of interest expense, interest income, and associated significant cash requirements; and the exclusion of depreciation and amortization, which represent unavoidable operating costs. Management compensates for the limitations of EBITDA by relying on our GAAP results. Our measure of EBITDA is not necessarily comparable to other similarly titled captions of other companies due to potential inconsistencies in the methods of calculation.The following is a reconciliation of net income to EBITDA for the consolidated company: Three Months EndedSeptember 30 Nine Months EndedSeptember 30 2016 2015 2016 2015 (millions)Net income$10.0 $22.0 $34.2 $49.9Interest expense7.1 5.7 19.4 16.8Interest income(0.1) (0.1) (0.2) (0.2)Change in fair value of interest rate swaps(0.8) — 0.8 —Loss on extinguishment of debt9.5 — 9.5 —Income tax provision5.5 12.6 19.2 28.8Depreciation and amortization19.5 14.2 53.2 41.1EBITDA$50.7 $54.5 $136.0 $136.4