Organization and Basis of Presentation - Summary of Financial Securities (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities	$ 128,502us-gaap_AvailableForSaleSecurities	$ 117,240us-gaap_AvailableForSaleSecurities
Total Amortized Cost	116,344ess_TotalAmortizedCost	106,955ess_TotalAmortizedCost
Total Gross Unrealized Gain Loss	7,158ess_TotalGrossUnrealizedGainLoss	5,285ess_TotalGrossUnrealizedGainLoss
Total Fair Value	123,502ess_TotalFairValue	112,240ess_TotalFairValue
Available-for-sale Securities and Held-to-maturity Securities	121,344	111,955
Available-For-Sale Securities And Held-To-Maturity Securities, Accumulated Gross Unrealized Gain (Loss), Before Tax	7,158	5,285
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	8,762	9,435
Available-for-sale Securities, Accumulated Gross Unrealized Gain (Loss), before Tax	176	145
Available-for-sale Securities	8,938	9,580
US Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	5,019	3,769
Available-for-sale Securities, Accumulated Gross Unrealized Gain (Loss), before Tax	7	3
Available-for-sale Securities	5,026	3,772
Common Stock [Member]
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale Securities, Amortized Cost Basis	31,753	25,755
Available-for-sale Securities, Accumulated Gross Unrealized Gain (Loss), before Tax	6,975	5,137
Available-for-sale Securities	38,728	30,892
Collateralized Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	70,810	67,996
Held-to-maturity Securities, Unrecognized Holding Gain	0	0
Held-to-maturity securities	70,810	67,996
Other Investments [Member]
Schedule of Available-for-sale Securities [Line Items]
Held-to-maturity Securities, Amortized Cost before Other than Temporary Impairment	5,000	5,000
Held-to-maturity Securities, Unrecognized Holding Gain	0	0
Held-to-maturity securities	$ 5,000	$ 5,000

Available-For-Sale Securities And Held-To-Maturity Securities, Accumulated Gross Unrealized Gain (Loss), Before Tax

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Total cost of debt and equity securities, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

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