11. Income Taxes:

The components of the income tax provision (benefit) are as follows (in thousands):


	December 31,
	2017			2016		2015
Federal:
Current	$	130,053		$	166,758	$	168,596
Deferred		(63,038	)		15,596		12,866

		67,015			182,354		181,462

State:
Current		20,070			4,296		20,948
Deferred		2,965			2,553		1,628

		23,035			6,849		22,576

Total	$	90,050		$	189,203	$	204,038

The Company files its tax return on a consolidated basis with its subsidiaries. The remaining affiliated professional contractors file tax returns on an individual basis.

The effective tax rate was 21.94%, 36.80% and 37.76% for the years ended December 31, 2017, 2016 and 2015, respectively. During the three months ended December 31, 2017, the Company recorded a $70.0 million income tax benefit related to the reduction of its net deferred tax liability resulting from the reduction in the corporate tax rate enacted in December 2017 under the Tax Cuts and Jobs Act of 2017. Excluding this favorable impact, our effective income tax rate for the year ended December 31, 2017 was 39.00%. During the three months ended September 30, 2016, the Company settled a certain tax matter with a taxing authority. In connection with this settlement, the Company’s effective income tax rate was favorably impacted by $10.6 million. Excluding this favorable impact, the effective tax rate for December 31, 2016 was 38.87%, as compared to 37.76% for 2015.

The differences between the effective rate and the United States federal income tax statutory rate are as follows:


	December 31,
	2017			2016			2015
Tax at statutory rate		35.00	%		35.00	%		35.00	%
State income tax, net of federal benefit		3.33			2.94			2.97
Non-deductible expenses		0.49			0.43			0.34
Change in accrual estimates relating to uncertain tax positions		0.02			(2.11	)		(0.43	)
Change in valuation allowance		—			0.48			0.29
Other, net		0.16			0.06			(0.41	)
Change in tax law		(17.06	)		—			—

Income tax provision		21.94	%		36.80	%		37.76	%

All of the Company’s deferred tax assets and liabilities are classified as long-term. The significant components of deferred income tax assets and liabilities are as follows (in thousands):


	December 31,
	2017			2016
Allowance for uncollectible accounts	$	80,056		$	103,525
Reserves and accruals		45,454			62,212
Stock-based compensation		7,975			13,726
Net operating loss carryforward		28,569			49,156
Property and equipment		685			1,621
Other		970			1,610

Deferred tax assets before valuation allowance		163,709			231,850
Less: Valuation allowance		(2,615	)		(4,014	)

Deferred tax assets, net of valuation allowance		161,094			227,836

Gross deferred tax liabilities:
Amortization		(258,618	)		(380,594	)
Accrual to cash adjustment		(31,290	)		(53,771	)
Other		(4,150	)		(1,023	)

Total deferred tax liabilities		(294,058	)		(435,388	)

Net deferred tax liability	$	(132,964	)	$	(207,552	)

The Company’s net deferred tax liability decreased by $74.6 million during 2017 of which $70.0 million is related to a reduction in the corporate tax rate enacted under the Tax Cuts and Jobs Act of 2017.

Beginning January 1, 2017, excess tax benefits or deficiencies associated with the exercise of stock options, the vesting of restricted and deferred stock and the purchase of shares under the Company’s non-qualified employee stock purchase plan are recognized as income tax benefits or expenses in the income statement in the reporting period in which they occur instead of an increase or decrease to additional paid-in-capital. For the year ended December 31, 2017, income tax expense of $0.2 million was recognized for excess tax deficiencies. For the years ended December 31, 2016 and 2015, respectively, additional paid-in-capital was increased by $4.2 million and $11.6 million for excess tax benefits.

The Company has net operating loss carryforwards for federal and state tax purposes totaling $116.0 million, $130.0 million, and $136.6 million at December 31, 2017, 2016 and 2015, respectively, expiring at various times in 2019 through 2037.

As of December 31, 2017, 2016 and 2015, the Company’s liability for uncertain tax positions, excluding accrued interest and penalties, was $11.0 million, $9.5 million and $18.4 million, respectively. As of December 31, 2017, the Company had $10.7 million of uncertain tax positions that, if recognized, would favorably impact its effective tax rate.

The following table summarizes the activity related to the Company’s liability for uncertain tax positions for the years ended December 31, 2017, 2016 and 2015 (in thousands):


	Years Ended December 31,
	2017			2016			2015
Balance at beginning of year	$	9,469		$	18,447		$	17,165
Increases related to prior year tax positions		2,284			301			467
Decreases related to prior year tax positions		(143	)		(3,927	)		(1,168	)
Increases related to current year tax positions		1,430			2,258			3,675
Settlements		—			(5,644	)		—
Decreases related to lapse of statutes of limitation		(2,068	)		(1,966	)		(1,692	)

Balance at end of year	$	10,972		$	9,469		$	18,447

During the year ended December 31, 2017, the Company increased its liability for uncertain tax positions by a total of $1.5 million, primarily related to additional taxes on current and prior year positions, partially offset by the expiration of statutes of limitation. During the year ended December 31, 2016, the Company decreased its liability for uncertain tax positions by a total of $9.0 million, primarily related to the settlement of a certain tax matter with a taxing authority and the expiration of statutes of limitation, partially offset by additional taxes on current year positions.

In addition, the Company anticipates that its liability for uncertain tax positions will be decreased by $1.1 million related to the expiration of certain statutes of limitation over the next 12 months.

The Company includes interest and penalties related to income tax liabilities in income tax expense. The Company recognized an income tax benefit of $0.2 million and $7.9 million and income tax expense of $0.6 million related to interest and penalties during the years ended December 31, 2017, 2016 and 2015, respectively. At December 31, 2017 and 2016, the Company’s accrued liability for interest and penalties related to income tax liabilities totaled $1.2 million and $1.4 million, respectively.

The Company is currently subject to U.S. Federal and various state income tax examinations for the tax years 2013 through 2016.