Accumulated Other Comprehensive Income (Loss) (Components of AOCI) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 3,427,741	$ 3,122,950	$ 2,856,111
Net unrealized gains (losses) arising during the period	15,284	(27,200)	4,215
Amount reclassified from AOCI	(4,657)	(6,005)	(23,393)
Other comprehensive income (loss)	10,627	(33,205)	(19,178)
Ending balance	3,841,951	3,427,741	3,122,950
Available- for-Sale Investment Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(28,772)	(6,144)	4,237
Net unrealized gains (losses) arising during the period	2,531	(16,623)	13,012
Amount reclassified from AOCI	(4,657)	(6,005)	(23,393)
Other comprehensive income (loss)	(2,126)	(22,628)	(10,381)
Ending balance	(30,898)	(28,772)	(6,144)
Foreign Currency Translation Adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(19,374)	(8,797)	0
Net unrealized gains (losses) arising during the period	12,753	(10,577)	(8,797)
Amount reclassified from AOCI	0	0	0
Other comprehensive income (loss)	12,753	(10,577)	(8,797)
Ending balance	(6,621)	(19,374)	(8,797)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(48,146)	(14,941)	4,237
Other comprehensive income (loss)	10,627	(33,205)	(19,178)
Ending balance	$ (37,519)	$ (48,146)	$ (14,941)