Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Statement of Comprehensive Income [Abstract]
Net income	$ 246,019	$ 232,940	$ 222,539
Available for sale securities:
Unrealized gains (losses) arising during the period	1,301	(29,817)	(20,860)
Income tax (expense) benefit related to unrealized gains (losses)	(503)	11,558	8,031
Reclassification adjustment for net realized gains in earnings	(27)	(858)	(2,922)
Income tax expense related to realized gains	10	332	1,125
Other comprehensive income (loss), net of tax	781	(18,785)	(14,626)
Comprehensive income	$ 246,800	$ 214,155	$ 207,913

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income. Excludes changes in equity resulting from investments by owners and distributions to owners.

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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