Long-Term Debt - Additional Information (Detail) - USD ($)		9 Months Ended
		Sep. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Document Period End Date		Sep. 30, 2020
Long-term debt including current maturities		$ 536,500,000	$ 486,600,000
Debt Instrument, Unused Borrowing Capacity, Amount		310,000,000
NYL Unsecured Debt [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities		$ 40,000,000.0
Debt Instrument, Interest Rate, Stated Percentage		2.96%
Prudential Unfunded Commitments [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities		$ 50,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	[1],[2]	3.00%
Uncollateralized Senior Note Due December Two Thousand Thirty Four [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage		2.98%
Term Note Due January Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities		$ 70,000,000	70,000,000
Term Note Due February Two Thousand Twenty [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities		0	30,000,000
3.48% note, due May 31, 2038 [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities		50,000,000	50,000,000
3.25% note, due April 30, 2032 [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities		70,000,000	$ 70,000,000
New York Life [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities		140,000,000
Aggregated Unfunded Commitments [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities		0
Aggregate Shelf Agreements [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities		360,000,000
MetLife [Member]
Debt Instrument [Line Items]
Senior notes	[3]	150,000,000.0
Debt Instrument, Unused Borrowing Capacity, Amount	[3]	150,000,000
Prudential [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities		220,000,000
Debt Instrument, Unused Borrowing Capacity, Amount	[1],[2]	150,000,000
New York Life [Member]
Debt Instrument [Line Items]
Debt Instrument, Unused Borrowing Capacity, Amount	[4]	10,000,000
Maximum [Member] | Aggregate Shelf Agreements [Member]
Debt Instrument [Line Items]
Senior notes		670,000,000
Maximum [Member] | Prudential [Member]
Debt Instrument [Line Items]
Senior notes	[1],[2]	370,000,000
Maximum [Member] | New York Life [Member]
Debt Instrument [Line Items]
Senior notes	[4]	$ 150,000,000

[1] 2) In April 2020, the Prudential Shelf Agreement was amended to increase the available borrowing capacity to $150.0 million. The Shelf Agreement expires in April 2023.

[2] (1) In January 2020, we requested and Prudential accepted our request to purchase $50.0 million of our unsecured debt. We issued the Shelf Notes in July 2020 at the rate of 3.00 percent per annum.

[3] (3) In May 2020, we reached into an agreement with MetLife to provide a new $150.0 million MetLife Shelf Agreement for a three-year term ending May 2023.

[4] (4) In February 2020, we requested and NYL accepted our request to purchase $40.0 million of our unsecured debt. The Shelf Notes were issued in August 2020 at the rate of 2.96 percent per annum. The Shelf Agreement expires in November 2021.