Parent Corporation Balance Sheets (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fixed income securities, available-for-sale, amortized cost	$ 5,879,986	$ 5,270,798
Preferred stock, par value per share	$ 0	$ 0	$ 0
Preferred stock, shares authorized	5,000,000	5,000,000	5,000,000
Preferred stock, shares issued	0	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value per share	$ 2	$ 2
Common stock, shares authorized	360,000,000	360,000,000
Common stock, shares issued	103,484,159	102,848,394
Treasury stock, shares	44,023,006	43,899,840
Parent Company [Member]
Fixed income securities, available-for-sale, amortized cost	$ 233,753	$ 111,208
Preferred stock, par value per share	$ 0	$ 0
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value per share	$ 2	$ 2
Common stock, shares authorized	360,000,000	360,000,000
Common stock, shares issued	103,484,159	102,848,394
Treasury stock, shares	44,023,006	43,899,840

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
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