Parent Corporation Balance Sheets (Details) - USD ($) $ / shares in Units, $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fixed income securities, available-for-sale, amortized cost	$ 5,270,798	$ 5,076,716
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	102,848,394	102,284,564
Treasury stock, shares	43,899,840	43,789,442
Parent Company [Member]
Fixed income securities, available-for-sale, amortized cost	$ 111,208	$ 89,799
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	102,848,394	102,284,564
Treasury stock, shares	43,899,840	43,789,442

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 1: http://fasb.org/us-gaap/role/ref/legacyRef
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