Long-Term Debt - Additional Information (Detail) - USD ($)		6 Months Ended
		Jun. 30, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Document Period End Date		Jun. 30, 2020
Long-term debt including current maturities		$ 446,500,000	$ 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	[1]	$ 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	[2],[3]	$ 50,000,000.0
Debt Instrument, Interest Rate, Stated Percentage	[2],[3]	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	[1]	100,000,000
Aggregated Unfunded Commitments [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities		90,000,000
Aggregate Shelf Agreements [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities		270,000,000
MetLife [Member]
Debt Instrument [Line Items]
Senior notes	[4]	150,000,000
Debt Instrument, Unused Borrowing Capacity, Amount	[4]	150,000,000
Prudential [Member]
Debt Instrument [Line Items]
Long-term debt including current maturities	[2],[3]	170,000,000
Debt Instrument, Unused Borrowing Capacity, Amount	[2],[3]	150,000,000.0
New York Life [Member]
Debt Instrument [Line Items]
Debt Instrument, Unused Borrowing Capacity, Amount	[1]	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	[2],[3]	370,000,000
Maximum [Member] | New York Life [Member]
Debt Instrument [Line Items]
Senior notes	[1]	$ 150,000,000

[1] ⁽⁴⁾ In February 2020, we requested and NYL accepted our request to purchase $40.0 million of our unsecured debt. We expect to issue the Shelf Notes in August 2020 at the rate of 2.96 percent per annum.

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

[3] ⁽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.

[4] ⁽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 in March 31, 2023.