ASSET RETIREMENT OBLIGATIONS, CONTRACTUAL THIRD-PARTY RECLAMATION RECEIVABLES, AND RECLAMATION DEPOSITS (Tables)

3 Months Ended

12 Months Ended

Mar. 31, 2014

Dec. 31, 2013

Asset Retirement Obligation Disclosure [Abstract]

Summary of asset retirement obligations, contractual third-party reclamation receivables and reclamation deposits

The asset retirement obligation, contractual third-party reclamation receivable, and reclamation deposits for each of the Company’s mines and ROVA at March 31, 2014 are summarized below:

	Asset Retirement Obligation		Contractual Third-Party Reclamation Receivable		Reclamation Deposits
	(In thousands)
Rosebud	$	127,989	$	21,656	$	75,315
Jewett	74,247		74,247		—
Absaloka	37,286		346		—
Beulah	17,764		—		—
Kemmerer	17,569		—		—
Savage	5,499		—		—
ROVA	909		—		—
Total	$	281,263	$	96,249	$	75,315

The asset retirement obligation, contractual third-party reclamation receivable, and reclamation deposits for each of the Company’s mines and ROVA at December 31, 2013 are summarized below:

	Asset Retirement Obligation		Contractual Third-Party Reclamation Receivable		Reclamation Deposits
	(In thousands)
Rosebud	$	126,965	$	21,186	$	74,921
Jewett	75,267		75,267		—
Absaloka	36,401		337		—
Beulah	17,785		—		—
Kemmerer	17,174		—		—
Savage	5,380		—		—
ROVA	892		—		—
Total	$	279,864	$	96,790	$	74,921

Changes in Company's asset retirement obligations

Changes in the Company’s asset retirement obligations were as follows:

	Three Months Ended March 31,
	2014			2013
	(In thousands)
Asset retirement obligations, beginning of year (including current portion)	$	279,864		$	263,847
Accretion	5,836			5,481
Liabilities settled	(4,437		)	(5,839		)
Asset retirement obligations, end of period	281,263			263,489
Less current portion	(22,227		)	(23,154		)
Asset retirement obligations, less current portion	$	259,036		$	240,335

Changes in the Company’s asset retirement obligations were as follows:

	Years Ended December 31,
	2013			2012
	(In thousands)
Asset retirement obligations, beginning of year (including current portion)	$	263,847		$	247,478
Accretion	21,905			21,909
Liabilities settled	(21,630		)	(17,342		)
Changes due to amount and timing of reclamation	15,742			(7,575		)
ARO acquired	—			19,377
Asset retirement obligations, end of year	279,864			263,847
Less current portion	(23,353		)	(22,238		)
Asset retirement obligations, less current portion	$	256,511		$	241,609

Carrying value and estimated fair value of reclamation deposits

The Company’s carrying value and estimated fair value of its reclamation deposits at March 31, 2014 are as follows:

	Carrying Value		Fair Value		Fair Value Hierarchy
	(In thousands)
Cash and cash equivalents	$	49,627	$	49,627	Level 1
Held-to-maturity securities	25,688		26,401		Level 2
	$	75,315	$	76,028

The Company’s carrying value and estimated fair value of its reclamation deposits at December 31, 2013 are as follows:

	Carrying Value		Fair Value
	(In thousands)
Cash and cash equivalents	$	63,525	$	63,525
Held-to-maturity securities	11,396		12,086
	$	74,921	$	75,611

Amortized cost, gross unrealized holding gains and losses and fair value of held-to-maturity securities

The amortized cost, gross unrealized holding gains and fair value of held-to-maturity securities at March 31, 2014 is as follows (in thousands):

Amortized cost	$	34,548
Gross unrealized holding gains	440
Gross unrealized holding losses	(281		)
Fair value	$	34,707

The amortized cost, gross unrealized holding gains and losses and fair value of held-to-maturity securities are as follows:

	December 31,
	2013			2012
	(In thousands)
Amortized cost	$	32,184		$	3,983
Gross unrealized holding gains	309			356
Gross unrealized holding losses	(447		)	—
Fair value	$	32,046		$	4,339

The amortized cost, gross unrealized holding gains and losses and fair value of held-to-maturity securities are as follows:

	Years Ended December 31,
	2013			2012
	(In thousands)
Amortized cost	$	11,396		$	19,832
Gross unrealized holding gains	764			1,190
Gross unrealized holding losses	(74		)	(9		)
Fair value	$	12,086		$	21,013

Schedule of maturities of held-to-maturity investments

Maturities of held-to-maturity securities are as follows at March 31, 2014:

	Amortized Cost		Fair Value
	(In thousands)
Due within one year	$	1,193	$	1,217
Due in five years or less	19,539		19,752
Due after five years to ten years	6,718		6,717
Due in more than ten years	7,098		7,021
	$	34,548	$	34,707

Maturities of held-to-maturity securities are as follows at March 31, 2014:

	Amortized Cost		Fair Value
	(In thousands)
Within one year	$	479	$	468
Due in five years or less	17,562		17,880
Due after five years to ten years	4,869		5,023
Due in more than ten years	2,778		3,030
	$	25,688	$	26,401

Maturities of held-to-maturity securities are as follows at December 31, 2013:

	Amortized Cost		Fair Value
	(In thousands)
Due within one year	$	1,111	$	1,143
Due in five years or less	17,839		17,944
Due after five years to ten years	6,769		6,665
Due in more than ten years	6,465		6,294
	$	32,184	$	32,046

Maturities of held-to-maturity securities are as follows at December 31, 2013:

	Amortized Cost		Fair Value
	(In thousands)
Within one year	$	17	$	17
Due in five years or less	6,645		6,976
Due after five years to ten years	3,002		3,123
Due in more than ten years	1,732		1,970
	$	11,396	$	12,086

Cost basis, gross unrealized holding gains and fair value of available-for-sale securities

The cost basis, gross unrealized holding gains and fair value of available-for-sale securities are as follows:

	December 31,
	2013		2012
	(In thousands)
Cost basis	$	—	$	1,000
Gross unrealized holding gains	—		39
Fair value	$	—	$	1,039

Held To Maturity Reclamation Deposits Table

The amortized cost, gross unrealized holding gains and losses and fair value of held-to-maturity securities at March 31, 2014 are as follows (in thousands):

Amortized cost	$	25,688
Gross unrealized holding gains	826
Gross unrealized holding losses	(113		)
Fair value	$	26,401