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Notes Payable - Additional Information (Detail) - USD ($)			1 Months Ended	12 Months Ended
	Nov. 05, 2015	Mar. 17, 2014	Apr. 30, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Feb. 09, 2016	Apr. 21, 2011
Debt Instrument [Line Items]
Consolidated Fixed Charge Coverage Ratio				2.50%
Consolidated Fixed Charge Coverage Ratio, actual				4.11%
Consolidated Leverage Ratio				2.75%
Consolidated Leverage Ratio, actual				1.47%
Line of credit facility covenants tangible net worth description				Permit Tangible Net Worth, calculated as of the last day of each fiscal quarter, to be less than the sum of (i) $229.0 million, plus (ii) 25% of net income for such fiscal quarter subsequent to December 31, 2010, plus (iii) 90% of the net cash proceeds from the issuance of the Company’s capital stock after December 31, 2010.
Tangible Net Worth sum				$ 366,800,000
Consolidated Net Income				25.00%
Percentage of net cash proceeds from issuance of capital stock				90.00%
Actual tangible net worth				$ 536,500,000
Principal payments on Series A senior notes				20,000,000	$ 20,000,000	$ 20,000,000
Senior notes principal balance outstanding				$ 298,603,000	303,473,000
Stated formula value for determining available facility amount								$ 250,000,000
2016 Amendment [Member]
Debt Instrument [Line Items]
Senior notes description				(i) the issuance period for the shelf notes to be issued and sold pursuant to the Note Purchase Agreement is extended until the earlier of February 9, 2019 or the termination of the issuance and sale of the shelf notes upon the 30 days’ prior notice of either PIM or the Company, and (ii) the definition of the “Available Facility Amount,” which is the aggregate amount of the shelf notes that may be authorized for purchase pursuant to the Note Purchase Agreement was amended to equal a formula based on: $250 million, minus the aggregate principal amount of the shelf notes then outstanding and purchased pursuant to the Note Purchase Agreement, minus the shelf notes accepted by the Company for purchase, but not yet purchased, by the Purchaser pursuant to the Note Purchase Agreement; provided, however, the aggregate amount of the shelf notes purchased by any corporation or other entity controlling, controlled by, or under common control with, PIM shall not exceed $200 million.
Series A senior notes [Member] \| 4.03% Series A senior notes due in 2018 [Member]
Debt Instrument [Line Items]
Aggregate principal amount									$ 100,000,000
Senior notes interest rate				4.03%					4.03%
Debt instrument, payment terms				Interest on these notes was due semi-annually in arrears and the principal was due in five equal annual installments, with the first payment due on April 21, 2014.
Debt instrument, maturity date				Apr. 21, 2018
Debt instrument, date of first required payment				Apr. 21, 2014
Principal payments on Series A senior notes			$ 20,000
Senior notes principal balance outstanding				$ 0	20,000,000
Shelf Notes [Member]
Debt Instrument [Line Items]
Aggregate principal amount				$ 100,000
Series B Senior Notes [Member] \| 3.68% Series B senior notes due in 2021 [Member]
Debt Instrument [Line Items]
Senior notes interest rate		3.68%		3.68%
Debt instrument, maturity date				Mar. 17, 2021
Senior notes principal balance outstanding				$ 40,000,000	40,000,000
Proceeds from issuance of senior notes		$ 40,000,000
Senior notes description				The Series B Senior Notes are an unsecured obligation of the Company, bear interest at a rate of 3.68% per annum and mature on March 17, 2021. Interest on the Series B Senior Notes is payable semi-annually beginning on September 17, 2014 and continuing thereafter on March 17 and September 17 of each year until maturity.
Senior notes interest payment				Semi-annually
Series C Senior Notes [Member] \| 3.84% Series C senior notes due in 2022 [Member]
Debt Instrument [Line Items]
Senior notes interest rate	3.84%			3.84%
Debt instrument, payment terms				Interest on the Series C Senior Notes is payable semi-annually beginning on May 5, 2016 and continuing thereafter on November 5 and May 5 of each year until maturity.
Debt instrument, maturity date				Nov. 05, 2022
Debt instrument, date of first required payment				May 05, 2016
Senior notes principal balance outstanding				$ 60,000,000	$ 60,000,000
Proceeds from issuance of senior notes	$ 60,000,000
Senior notes interest payment				Semi-annually
Minimum [Member]
Debt Instrument [Line Items]
Tangible Net Worth sum				$ 229,000,000
Maximum [Member]
Debt Instrument [Line Items]
Aggregate principal amount								$ 200,000,000
Maximum [Member] \| Shelf Notes [Member]
Debt Instrument [Line Items]
Senior notes maturity period				12 years
Senior notes average life				10 years
Credit Facility [Member]
Debt Instrument [Line Items]
Revolving credit facility, maturity period				5 years
Revolving credit facility, maturity date				Mar. 31, 2021
Credit facility, maximum				$ 420,000,000
Revolving credit facility, available maximum borrowing capacity				620,000,000
Additional commitments				$ 200,000,000
Consolidated Fixed Charge Coverage Ratio				2.50%
Consolidated Fixed Charge Coverage Ratio, actual				4.11%
Consolidated Leverage Ratio				2.75%
Consolidated Leverage Ratio, actual				1.47%
Line of credit facility covenants tangible net worth description				Permit Tangible Net Worth as of the end of any fiscal quarter of the Company to be less than the sum of (i) $246.1 million plus (ii) 25% of the Company’s Consolidated Net Income (as defined in the Amended Credit Facility) (but only if a positive number) for each fiscal quarter ended subsequent to December 31, 2011
Tangible Net Worth sum				$ 366,800,000
Consolidated Net Income				25.00%
Percentage of net cash proceeds from issuance of capital stock				90.00%
Actual tangible net worth				$ 536,500,000
Debt instrument description of variable rate basis				(i) LIBOR plus a defined margin, or (ii) the Agent bank’s prime rate (“base rate”) plus a margin. The applicable margin for each type of loan is measured based upon the Consolidated Leverage Ratio at the end of the prior fiscal quarter and ranges from 1.00% to 1.75% for LIBOR loans and 0% to 0.75% for base rate loans. In addition, the Company pays an unused commitment fee for the portion of the $420.0 million credit facility that is not used. These fees are based upon the Consolidated Leverage Ratio and range from 0.15% to 0.30%. As of December 31, 2018 and 2017, the applicable margins were 1.25% for LIBOR based loans, 0.25% for base rate loans and 0.20% for unused fees. Amounts borrowed under the Sweep Service Facility are based upon the MUFG Union Bank, N.A. base rate plus an applicable margin and an unused commitment fee for the portion of the $12.0 million facility not used.
Unused commitment fees				$ 420,000,000
Credit Facility [Member] \| Unused Fees [Member]
Debt Instrument [Line Items]
Debt instrument interest rate at period end				0.20%	0.20%
Credit Facility [Member] \| LIBOR Loans [Member]
Debt Instrument [Line Items]
Debt instrument interest rate at period end				1.25%	1.25%
Credit Facility [Member] \| Base Rate Loans [Member]
Debt Instrument [Line Items]
Debt instrument interest rate at period end				0.25%	0.25%
Credit Facility [Member] \| Sweep Service Facility [Member]
Debt Instrument [Line Items]
Credit facility, maximum					$ 432,000,000
Revolving credit facility, available maximum borrowing capacity					233,400,000
Amount outstanding					$ 198,600,000
Unused commitment fees				$ 12,000,000
Credit Facility [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Tangible Net Worth sum				$ 246,100,000
Leverage ratio				0.15%
Credit Facility [Member] \| Minimum [Member] \| LIBOR Loans [Member]
Debt Instrument [Line Items]
Leverage ratio				1.00%
Credit Facility [Member] \| Minimum [Member] \| Base Rate Loans [Member]
Debt Instrument [Line Items]
Leverage ratio				0.00%
Credit Facility [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Leverage ratio				0.30%
Credit Facility [Member] \| Maximum [Member] \| LIBOR Loans [Member]
Debt Instrument [Line Items]
Leverage ratio				1.75%
Credit Facility [Member] \| Maximum [Member] \| Base Rate Loans [Member]
Debt Instrument [Line Items]
Leverage ratio				0.75%
Unsecured revolving credit facility before Renewal [Member]
Debt Instrument [Line Items]
Credit facility, maximum				$ 420,000,000
Letters of Credit [Member]
Debt Instrument [Line Items]
Credit facility, maximum				25,000,000
Letters of Credit [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Credit facility, maximum							$ 10,000,000
Letters of Credit [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Credit facility, maximum							$ 12,000,000
Swingline Loans [Member]
Debt Instrument [Line Items]
Credit facility, maximum				$ 10,000,000