Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Statement of Comprehensive Income [Abstract]
Net income	$ 58,368	$ 120,867	$ 88,356	$ 149,654
Unrealized (losses) gains on investments available-for-sale:
Unrealized holding (losses) gains arising during the period	(31,186)	23,520	(14,904)	51,908
Less: Reclassification adjustments for gains included in earnings	(1,564)	(1,928)	(2,169)	(2,236)
Unrealized (losses) gains on investments available-for-sale, net	(32,750)	21,592	(17,073)	49,672
Defined benefit pension plans:
Prior service cost arising during the period	0	0	0	0
Net loss arising during the period	0	0	0	0
Less: Amortization of prior service cost and net loss included in net periodic pension cost	640	150	1,280	300
Defined benefit pension plans, net	640	150	1,280	300
Other comprehensive (loss) income before tax	(32,110)	21,742	(15,793)	49,972
Income tax (benefit) expense related to components of other comprehensive income	(11,273)	7,683	(5,515)	17,556
Other comprehensive (loss) income, net of tax	(20,837)	14,059	(10,278)	32,416
Comprehensive income	$ 37,531	$ 134,926	$ 78,078	$ 182,070