Debt (Schedule Of Obligations Under Debt Arrangements) (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Sep. 30, 2015	Jul. 23, 2015	Dec. 31, 2014		May. 15, 2014	Feb. 08, 2013	Feb. 01, 2012
Debt Instrument [Line Items]
Senior secured credit facility		$ 1,115,000			$ 550,400
Unamortized Debt Issuance Expense and Discounts (Premiums)		42,946			19,111
SENIOR UNSECURED NOTES, net of debt issuance costs		1,807,054			1,030,889
Long-term debt, Face Values		2,965,000			1,600,400
Total long-term debt		2,922,054			1,581,289
Unamortized premium		0			639
Seven Point Eight Seventy Five Percentage Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense and Discounts (Premiums)		0			4,502	[1]
SENIOR UNSECURED NOTES, net of debt issuance costs		0			345,498
Debt Instrument, Face Amount		0			350,000				$ 100,000
Six Percentage Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense and Discounts (Premiums)		7,825			0
SENIOR UNSECURED NOTES, net of debt issuance costs		392,175	$ 400,000		0
Debt Instrument, Face Amount		400,000			0
Five Point Seventy Five Percentage Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense and Discounts (Premiums)	[2]	5,183			6,202
SENIOR UNSECURED NOTES, net of debt issuance costs		344,817			343,798
Debt Instrument, Face Amount		350,000			350,000			$ 350,000
Five Point Six Two Five Percentage Senior Unsecured Notes [Member] [Domain]
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense and Discounts (Premiums)		7,510			8,407
SENIOR UNSECURED NOTES, net of debt issuance costs		342,490			341,593
Debt Instrument, Face Amount		350,000			350,000		$ 350,000
Six Point Seven Five Percentage Senior Unsecured Notes [Member]
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense and Discounts (Premiums)		22,428			0
SENIOR UNSECURED NOTES, net of debt issuance costs		727,572			0
Debt Instrument, Face Amount		750,000		$ 750,000	0
Credit Facility [Domain]
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense and Discounts (Premiums)		$ 0			$ 0

[1] (2)Net of unamortized premium of $639 in 2014.

[2] (1)In April 2015, the FASB issued guidance that requires the presentation of debt issuance costs in financial statements as a direct reduction of related debt liabilities with amortization of debt issuance costs reported as interest expense. Under current U.S. GAAP standards, debt issuance costs are reported as deferred charges (i.e., as an asset). This guidance is effective for annual periods, and interim periods within those fiscal years, beginning after December 15, 2015 and is to be applied retrospectively upon adoption. Early adoption is permitted, including adoption in an interim period for financial statements that have not been previously issued. Genesis adopted this guidance in the fourth quarter of 2015.