Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Assets
Available-for-sale securities, amortized cost	$ 2,059,445	$ 2,124,567
Shareholders' Equity
Common stock, par value (in dollars per share)	$ 2.50
Common stock, shares authorized (in shares)	500,000,000
Common stock, shares issued (in shares)	94,162,728	96,254,903