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Notes Payable - Additional Information (Detail) - USD ($) |  |  |  |  | 12 Months Ended |  |
--- | --- | --- | --- | --- | --- | --- | ---
Oct. 01, 2020 | Mar. 31, 2020 | Nov. 05, 2015 | Mar. 17, 2014 | Dec. 31, 2020 | Dec. 31, 2019 | Mar. 31, 2016
Debt Instrument [Line Items] |  |  |  |  |  |  |
Consolidated Fixed Charge Coverage Ratio |  |  |  |  | 2.50% |  |
Consolidated Fixed Charge Coverage Ratio, actual |  |  |  |  | 4.34% |  |
Consolidated Leverage Ratio |  |  |  |  | 275.00% |  |
Consolidated Leverage Ratio, actual |  |  |  |  | 92.00% |  |
Senior notes principal balance outstanding |  |  |  |  | $ 222,771,000 | $ 293,459,000 |
Note Purchase Agreement [Member] | Notes Issued March 17, 2014 and Due March 17, 2021 [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Proceeds from issuance of senior notes |  | $ 40,000,000.0 |  |  |  |  |
Senior notes interest rate |  | 3.68% |  |  |  |  |
Debt instrument, maturity date |  |  |  |  | Mar. 17, 2021 |  |
Note Purchase Agreement [Member] | Notes Issued November 5, 2015 and Due November 5, 2022 [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Proceeds from issuance of senior notes |  | $ 60,000,000.0 |  |  |  |  |
Senior notes interest rate |  | 3.84% |  |  |  |  |
Debt instrument, maturity date |  |  |  |  | Nov. 05, 2022 |  |
Shelf Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Proceeds from issuance of senior notes |  | $ 250,000,000 |  |  |  |  |
Senior notes description |  |  |  |  | In addition, pursuant to the Note Purchase Agreement, the Company may authorize the issuance and sale of additional senior notes (the “Shelf Notes”) in the aggregate principal amount of (x) $250 million minus (y) the amount of other notes (such as the Series B Senior Notes and Series C Senior Notes, each defined below) then outstanding, to be dated the date of issuance thereof, to mature, in case of each Shelf Note so issued, no more than 15 years after the date of original issuance thereof, to have an average life, in the case of each Shelf Note so issued, of no more than 15 years after the date of original issuance thereof, to bear interest on the unpaid balance thereof from the date thereof at the rate per annum, and to have such other particular terms, as shall be set forth, in the case of each Shelf Note so issued, in accordance with the Note Purchase Agreement. |  |
Rate Lock Agreement [Member] | Senior Unsecured Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Proceeds from issuance of senior notes | $ 40,000,000.0 |  |  |  |  |  |
Senior notes interest rate | 2.57% |  |  |  |  |  |
Debt instrument, maturity date |  |  |  |  | Mar. 17, 2021 |  |
Debt instrument, term | 7 years |  |  |  |  |  |
3.68% Series B senior notes due in 2021 [Member] | Series B Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Proceeds from issuance of senior notes |  |  |  | $ 40,000,000.0 |  |  |
Senior notes interest rate |  |  |  | 3.68% | 3.68% |  |
Debt instrument, maturity date |  |  |  |  | Mar. 17, 2021 |  |
Senior notes description |  |  |  |  | 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 |  |
Senior notes principal balance outstanding |  |  |  |  | $ 40,000,000 | 40,000,000 |
3.84% Series C senior notes due in 2022 [Member] | Series C Senior Notes [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Proceeds from issuance of senior notes |  |  | $ 60,000,000.0 |  |  |  |
Senior notes interest rate |  |  | 3.84% |  | 3.84% |  |
Debt instrument, maturity date |  |  |  |  | Nov. 05, 2022 |  |
Senior notes description |  |  |  |  | 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. |  |
Senior notes interest payment |  |  |  |  | semi-annually |  |
Senior notes principal balance outstanding |  |  |  |  | $ 60,000,000 | $ 60,000,000 |
Debt instrument, date of first required payment |  |  |  |  | May 05, 2016 |  |
Credit Facility [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Credit facility, maximum |  | 420,000,000.0 |  |  |  |  |
Revolving credit facility, maturity date |  |  |  |  | Mar. 31, 2025 |  |
Additional senior notes indebtedness |  | 250,000,000.0 |  |  |  |  |
Consolidated Fixed Charge Coverage Ratio |  |  |  |  | 2.50% |  |
Consolidated Fixed Charge Coverage Ratio, actual |  |  |  |  | 4.34% |  |
Consolidated Leverage Ratio |  |  |  |  | 275.00% |  |
Consolidated Leverage Ratio, actual |  |  |  |  | 92.00% |  |
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, 2020 and 2019, 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 |  |  |  |  | $ 420,000,000.0 |  |
Credit Facility [Member] | Unused Fees [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt instrument interest rate at period end |  |  |  |  | 0.20% | 0.20% |
Credit Facility [Member] | Minimum [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Leverage ratio |  |  |  |  | 0.15% |  |
Credit Facility [Member] | Maximum [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Leverage ratio |  |  |  |  | 0.30% |  |
Credit Facility [Member] | LIBOR Loans [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Debt instrument interest rate at period end |  |  |  |  | 1.25% | 1.25% |
Credit Facility [Member] | LIBOR Loans [Member] | Minimum [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Leverage ratio |  |  |  |  | 1.00% |  |
Credit Facility [Member] | LIBOR Loans [Member] | Maximum [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Leverage ratio |  |  |  |  | 1.75% |  |
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] | Base Rate Loans [Member] | Minimum [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Leverage ratio |  |  |  |  | 0.00% |  |
Credit Facility [Member] | Base Rate Loans [Member] | Maximum [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Leverage ratio |  |  |  |  | 0.75% |  |
Credit Facility [Member] | MUFG Union Bank, N.A [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Credit facility, maximum |  | 12,000,000.0 |  |  |  |  |
Revolving credit facility, maturity date |  |  |  |  | Mar. 31, 2025 |  |
Credit Facility [Member] | Treasury Sweep Note Due March 31, 2025 [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Credit facility, indebtedness |  | 12,000,000.0 |  |  |  |  |
Revolving credit facility, maturity date |  |  |  |  | Mar. 31, 2025 |  |
Credit Facility [Member] | Notes Issued March 17, 2014 and Due March 17, 2021 [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Revolving credit facility, maturity date |  |  |  |  | Mar. 17, 2021 |  |
Credit facility, aggregate outstanding principal |  | 40,000,000.0 |  |  |  |  |
Credit Facility [Member] | Notes Issued November 5, 2015 and Due November 5, 2022 [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Revolving credit facility, maturity date |  |  |  |  | Nov. 05, 2022 |  |
Credit facility, aggregate outstanding principal |  | 60,000,000.0 |  |  |  |  |
Credit Facility [Member] | Sweep Service Facility [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Credit facility, maximum |  |  |  |  | $ 432,000,000.0 |  |
Credit facility, aggregate outstanding principal |  |  |  |  | 122,800,000 |  |
Unused commitment fees |  |  |  |  | $ 12,000,000.0 |  |
Credit Facility [Member] | One or More Tranches [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Revolving credit facility, available maximum borrowing capacity |  | 670,000,000.0 |  |  |  |  |
Letters of Credit [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Credit facility, maximum |  | 25,000,000.0 |  |  |  |  |
Swingline Loans [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Credit facility, maximum |  | 10,000,000.0 |  |  |  |  |
Unsecured revolving credit facility before Renewal [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Credit facility, maximum |  | $ 420,000,000.0 |  |  |  |  |
Unsecured revolving credit facility before Renewal [Member] | MUFG Union Bank, N.A [Member] |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |
Credit facility, maximum |  |  |  |  |  |  | $ 12,000,000.0

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