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Investment Securities
 3 Months Ended
Mar. 31, 2024
Investments, Debt and Equity Securities [Abstract]  
Investment Securities Investment Securities.    Investment securities we hold are recorded at fair value in the accompanying consolidated balance sheets. We apply regulated operations accounting to the unrealized gains and losses of all investment securities. All realized and unrealized gains and losses are determined using the specific identification method.
The following tables summarize debt and equity securities as of March 31, 2024 and December 31, 2023.
Gross Unrealized
(dollars in thousands)
March 31, 2024CostGainsLossesFair
Value
Equity$350,947 $292,940 $(6,033)$637,854 
Debt903,715 2,574 (27,229)879,060 
Other7,549 1 (10)7,540 
Total$1,262,211 $295,515 $(33,272)$1,524,454 
Gross Unrealized
(dollars in thousands)
December 31, 2023CostGainsLossesFair
Value
Equity$344,669 $246,795 $(5,549)$585,915 
Debt908,316 3,938 (25,181)887,073 
Other2,889 61 (36)2,914 
Total$1,255,874 $250,794 $(30,766)$1,475,902 

The cost basis of our debt securities that were in unrealized loss positions at March 31, 2024 was $832,901,000. At March 31, 2024, $3,322,000 of the $27,229,000 of unrealized losses relates to securities that have been in unrealized loss positions for less than twelve months and $23,907,000 relates to securities that have been in unrealized loss positions for greater than twelve months. These unrealized losses are primarily attributable to increases in market interest rates.

The cost basis of our debt securities that were in unrealized loss positions at December 31, 2023 was $788,798,000. At December 31, 2023, $3,362,000 of the $25,181,000 of unrealized losses relates to securities that have been in unrealized loss positions for less than twelve months and $21,819,000 relates to securities that have been in unrealized loss positions for greater than twelve months. These unrealized losses are primarily attributable to increases in market interest rates.