Loans Payable - Additional Information (Detail)		2 Months Ended	12 Months Ended
	Dec. 24, 2014 USD ($)	Feb. 29, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)
Line of Credit Facility [Line Items]
Proceeds from loans			$ 657,700,000
Repayment of loans			377,500,000	$ 199,245,000	$ 39,505,000
Interest expense			$ 19,400,000	12,900,000	$ 12,400,000
Clarendon Facility
Line of Credit Facility [Line Items]
Repayment of outstanding principal balance				79,000,000
SeaBright Facility
Line of Credit Facility [Line Items]
Repayment of outstanding principal balance				$ 111,000,000
EGL Revolving Credit Facility | Revolving Credit Facility
Line of Credit Facility [Line Items]
Facility term (years)			5 years
Line of credit facility, maximum borrowing capacity			$ 665,000,000
Unused borrowing capacity available, amount			$ 159,200,000
Interest rate payable period, maximum (months)			6 months
Commitment fee on unused portion (percent)			35.00%
Percentage increase in interest rate if payment default occurs			1.00%
Debt Instrument, covenant compliance, consolidated indebtedness to total capitalization (no greater than)			0.35
Debt instrument, covenant compliance, net income (loss) threshold (not less than)			$ 1,500,000,000.0
Debt instrument, covenant compliance, net income threshold (percent) (not less than)			50.00%
Debt instrument, covenant compliance, percentage of proceeds of common stock (not less than)			75.00%
EGL Revolving Credit Facility | Revolving Credit Facility | LIBOR
Line of Credit Facility [Line Items]
Credit Facility, margin rate (percent)			2.75%
EGL Revolving Credit Facility | Revolving Credit Facility | Minimum
Line of Credit Facility [Line Items]
Credit Facility, margin rate (percent)			2.50%
EGL Revolving Credit Facility | Revolving Credit Facility | Maximum
Line of Credit Facility [Line Items]
Credit Facility, margin rate (percent)			3.25%
Sussex Facility
Line of Credit Facility [Line Items]
Facility term (years)	4 years		4 years
Line of credit facility, maximum borrowing capacity	$ 109,000,000
Interest rate payable period, maximum (months)				6 months
Percentage increase in interest rate if payment default occurs	1.00%
Percentage of consideration payable for acquisition	50.00%
Acquisition completion date				Jan. 27, 2015
Repayment of outstanding principal balance			$ 15,000,000
Debt instrument, covenant compliance, statutory surplus threshold, multiplier	1.1
Debt instrument, covenant compliance, outstanding loan balance threshold, multiplier	2
Sussex Facility | LIBOR
Line of Credit Facility [Line Items]
Credit Facility, margin rate (percent)				2.75%
Subsequent Event | Sussex Facility
Line of Credit Facility [Line Items]
Line of credit facility, maximum borrowing capacity		$ 73,500,000
Repayment of outstanding principal balance		$ 20,500,000