Significant Accounting Policies - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Minimum future rent on non cancelable lease due in 2015	$ 5,600,000
Minimum future rent on non cancelable lease due in 2016	5,800,000
Minimum future rent on non cancelable lease due in 2017	5,700,000
Minimum future rent on non cancelable lease due in 2018	5,600,000
Minimum future rent on non cancelable lease due in 2019	5,400,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment	1,295,000	0	0
Other than Temporary Impairment Losses, Investments, Portion in Other Comprehensive Loss, before Tax, Including Portion Attributable to Noncontrolling Interest, Available-for-sale Securities	1,500,000	2,200,000
Capitalized indirect costs incurred (less than for 2012)	600,000	700,000	100,000
Timber holdings valuation sample	20.00%
Fair Value	609,920,000
Adjustments for New Accounting Pronouncement
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Reduction to Liability for Uncertain Tax Positions, Noncurrent	1,700,000
Securities of any one issuer (excluding the U.S. Government)
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Available For Sale Securities, Preferred Stock, One Issuer	23,700,000
U.S. treasury securities
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Fair Value	509,800,000
Corporate debt securities
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Fair Value	$ 100,100,000	$ 22,000,000
Minimum
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Estimated useful life (in years)	3 years
Minimum | Real Estate
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Estimated useful life (in years)	5 years
Maximum
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Estimated useful life (in years)	30 years
Maximum | Real Estate
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Estimated useful life (in years)	40 years